Texas Sheet Cake with Watermelon White Chocolate Fudge Frosting

I grew up eating Texas sheet cake. At every block party, picnic, potluck, and barbecue, my mom would bring a tray of this fudgy chocolate delight. So of course, I had to experiment with a variation. This version, with watermelon white chocolate fudge frosting, brings a fun pop of both color and flavor to the classic.

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I’ve brought you interesting flavor combinations before, but don’t get scared away by this combination of chocolate and watermelon. Trust me, it just works. I was actually inspired by a pack of chocolate-covered, watermelon-flavored gummies from Trader Joe’s, so you know it’s gotta be a good match.

Whenever I’m trying something brand new with no real recipe, like the frosting on this cake, I like to pair it with a classic. Since Texas sheet cake itself is a simple recipe, it was easy to use it as the base for something just a little different. And the result was both delicious and maybe the most “Slumber & Scones” on-brand frosting I have ever made!

What You Need to Get Started

You can easily find all of the ingredients for this watermelon Texas sheet cake at your grocery store:

• Fridge: Butter, Eggs, Sour cream
• Pantry: Cocoa powder, Flour, Sugar, Baking soda, White chocolate chips, Confectioners sugar, Food coloring
• Produce: Watermelon
• Equipment: Jellyroll sheet pan, Parchment paper, Mixing bowls, Whisk, Flexible spatula, Muddler or blender, Mesh strainer, Stand mixer or hand mixer

Let’s Make Texas Sheet Cake

There are two reasons to make a Texas sheet cake instead of another type of chocolate cake. First, it’s super easy. Second, it’s fast. Plus, it’s baked on a sheet pan, it’s easy to cut and serve, you can store it well, and there’s no need for any fancy decorating.

Since this does come together pretty quickly, it’s best to preheat your oven right at the beginning. Make sure you have a rack in the center, and also line a sheet pan with parchment paper. Use a little nonstick spray to hold the paper to the pan, and then coat the top of the parchment as well.

It’s important that you are using a “jellyroll” style sheet pan. This means that it has sides, and is not a fully flat cookie sheet. They don’t need to be high sides, just about half to three-quarters of an inch.

Unlike most cakes where you cream together the butter and sugar, here you’ll start by melting the butter along with the cocoa powder and some water. You can do this on the stovetop using a double boiler if you prefer, but I think the easiest method is to use the microwave.

Add everything to a glass mixing bowl, and microwave it in thirty-second increments. At the end of each, give the mixture a stir. After a few blasts in the microwave, the butter will begin to melt, and you will be able to stir it into a smooth, liquidy chocolate.

Using cocoa only in this cake gives it a distinct taste. It’s chocolatey, but not overwhelming. The melted butter will help with the spongy texture of this dense sheet-pan cake.

Meanwhile, combine the dry ingredients in a separate medium bowl. Whisk together the flour, sugar, and baking soda. There’s a lot of sugar in the recipe. Whereas many cakes have equal parts flour and sugar by weight, here they are equal by volume. Since sugar is heavier than flour, that makes for a nice and sweet chocolate cake.

You’ll notice that there’s only baking soda and no baking powder or cream of tartar in the recipe. That’s because cocoa is acidic, and will be plenty to react with the baking soda, giving this cake just enough rise.

Once everything is whisked together, add the dry ingredients into the cocoa and butter mixture. Mix well to fully combine these together. Normally, when making cake, this step would warn you about overmixing. For this Texas sheet cake, don’t even worry about that.

Finally, add in the last two ingredients of the cake batter: the eggs and sour cream. Eggs provide both structure and rise. Sour cream adds both flavor and additional acidity to work with the baking soda. I always love adding some extra dairy to my cakes, and this recipe is probably where that all started for me.

You will also want to thoroughly mix these two ingredients into the rest of the batter. Again, there’s no need to focus on not overmixing here. This cake is meant to be made with a nice, uniformly mixed batter.

When it’s all mixed together, transfer the batter to your prepared sheet pan. Be sure to use your flexible spatula to get all that yummy chocolate goodness out of the bowl and into the pan. Then, spread it all around the pan and into the corners by tilting gently and using your spatula to guide it.

Bake the cake for right around twenty-five minutes on that center rack. When it’s ready, the cake will both look and feel spongy. It will have a spring to it when you gently touch the center, bouncing back. A toothpick will also come out clean.

Let the cake cool in the pan for at least ten minutes. Then you can carefully use the parchment paper to slide it onto a surface for frosting and cutting. You will want to wait until it’s cooled to serve, so if you prefer, you can leave the cake in the pan until after it is frosted then fully cooled.

How to Make Watermelon White Chocolate Frosting

While the cake is baking, prepare the frosting. There’s no time for downtime here, since this fudgey frosting gets added while the cake is still warm. Start by muddling the watermelon, or use a blender to get a puree. Then push that through a fine mesh strainer until you have half a cup of juice.

In a small saute pan or saucepan, add the juice and heat over medium until it is bubbling and boiling. It may separate a little bit, and that’s fine. Watermelon is mostly water, so we are trying to cook some of that off and make it a bit denser.

You will want to end up with about a third of a cup of the reduced juice. Check the amount by transferring the reduced juice back into a measuring cup. If you still have too much, then it needs more time. Add it back to the pan and continue boiling. Overall, it should take about five minutes or so.

Add the white chocolate chips to a small microwave-safe bowl. Melt these in the microwave in thirty-second increments, stirring after each until smooth. It’s similar to how you melted the butter for the cake. Once it’s smooth, you are ready to move on. Again, you can use a double boiler on the stove if you prefer.

Add the reduced watermelon juice to the melted white chocolate. Stir to thoroughly combine these two together, and this will become the base to flavor your frosting. Set the bowl aside for now while you start preparing the rest.

The other half of the frosting is similar to a simple buttercream. Add the butter to a bowl of a stand mixer fitted with the paddle attachment. If you prefer, you can use a hand mixer with the beater attachments to make this. Mix the butter alone on medium-low until it is lightened in color and fluffy.

Add in the confectioners sugar in half-cup batches. Along with each batch of sugar, also add in a splash of the watermelon-white chocolate mixture. The extra liquid will help the frosting come together more easily.

Mix to thoroughly incorporate each batch, until you are no longer seeing streaks of sugar. The frosting should be getting both thicker and pinker as you add the ingredients. With the last of the sugar, add in the rest of the watermelon-white chocolate as well. Mix it in thoroughly, and then check out the color. If you’d like, add food coloring at this point. I used one drop of red to help bring out a more pinkish tone.

When done, the frosting should be thick, but spreadable. It will hold its own shape while cool, but once you add it onto the warm cake, it will start to melt slightly. Add the watermelon fudge frosting onto the cake and spread to coat it completely.

It’s okay if some of the frosting runs down the sides of the cake. You will want to let the cake and frosting cool fully before cutting and serving it, and as it cools, any drips of frosting will harden. You can chill this in the fridge to cool it faster, if needed.

If you are making this in advance, store the frosted cake, sliced or unsliced, covered in the fridge for a few days. If you need longer storage, freeze tightly wrapped slices or the entire cake for a couple months. Thaw the cake in the fridge overnight before serving it.

How to Serve Watermelon Texas Sheet Cake

This cake is rich and sweet, making it the perfect dessert. Why not pair it with some tasty and complimentary drinks? A chocolate old fashioned will accent the chocolate cake, or a birthday cake martini is great to let the watermelon shine. If you are looking for something warmer, try a raspberry white hot chocolate, with or without booze.

You can also present this cake as part of a chocolate-themed dessert spread. Chocolate lovers will definitely appreciate other impressive cocoa-flavored treats. Serve this alongside slices of s’mores drip cake, peanut butter cup Rice Krispies treats, and a miso chocolate torte.

Or make a differently themed dessert spread focused on bright colors to highlight the pink frosting on this cake. Blueberry lemon bars, cherry cheesecake napoleons, and plum jam macarons are other pretty bites you are sure to love.

Use Up Leftover Ingredients

• Extra watermelon is the perfect excuse to enjoy a watermelon daiquiri on the rocks.
• Use up cocoa powder in super rich rocky road black bean brownies.
• Highlight white chocolate chips in a cozy chai white hot chocolate.
• Sour cream adds lots of moisture to a coffee cake with peaches.

Special Equipment

• Jellyroll sheet pan
• Parchment paper
• Mixing bowls
• Whisk
• Flexible spatula
• Muddler or blender
• Mesh strainer
• Stand mixer or hand mixer

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• Small pink plate
• Pink striped towel
• Metal mixing bowl
• Beaker measuring cups