About a month ago, I got a special request from Jeremy’s mom to come up with a recipe for a watermelon cake so she could make it for his younger brother’s birthday (he is a watermelon enthusiast). It was a tricky challenge, since watermelon doesn’t really taste like watermelon anymore once you heat it up, and most cakes involve baking. So, after much thought and one failed cake-making attempt, I decided to leave the watermelon out of the actual sponge cake itself, and instead focused on creating the watermelon flavors within the layers. I ended up making a cold-pressed watermelon simple syrup and then beating it into some heavy cream for a watermelon whipped cream. Then I mixed more of the syrup into a chilled curd to make a fresh and tasty watermelon curd. I prepared the sponge cake using a simple recipe I came across here, and spread the watermelon whipped cream and curd between each layer when assembling.
The end result of all this brainstorming and planning and tomfoolery was the single most refreshing and summery cake I’ve ever had. Bright, fresh, and cool; the light and airy spongecake was the perfect match for the rich fruity fillings. It sliced incredibly easy, and was altogether pretty easy to make. The most complex part was folding the egg whites into the batter, if you’ve never attempted this before I highly recommend taking a gander at this tutorial. Very helpful! The general purpose of folding in the egg whites in rather than stirring them is to keep all those nice tiny bubbles you whipped up intact, so that way when the cake bakes it retains that light and airy texture, rather than the more dense texture of traditional yellow cakes. The fluffiness of the cake is perfect for the heat of late, I hope it finds its way into your kitchen this summer, it is a real treat!
First, make the watermelon syrup. Mix together the watermelon puree and sugar in a medium sized bowl. Cover and refrigerate for 4 hours or overnight. Strain the syrup though a mesh sieve to remove any large particles and seeds. Reserve the syrup in a separate bowl and cover and refrigerate it until use. (You will have some left over after the recipe, I recommend using it as a mixer for cocktails or stirring it into some fresh lemon juice for a watermelon lemonade).
To make the watermelon curd, cream together the butter and sugar until smooth. Add the eggs, one at a time, and then the yolk, and mix until blended. Pour the mixture into a small thick-bottomed saucepan and cook over low heat whisking constantly, until the temperature of the mixture reaches about 170 degrees Fahrenheit with a candy thermometer, or until the mixture has thickened significantly (it usually takes between 8 and 15 minutes depending on your heat level and stovetop). Remove from heat immediately and continue whisking for the first few minutes so the mixture doesn’t cook onto the bottom of the hot pan. Allow to cool to room temperature, then whisk in 3 tablespoons of the chilled watermelon syrup and the optional pink food coloring until incorporated. Cover and place the curd in the refrigerator until use.
To make the cake, Preheat the oven to 325 degrees Fahrenheit. Line the bottom of (3) 8-inch cake pans with parchment paper. Set aside. In a large bowl, beat the egg yolks at medium-high speed until thick, about 2 minutes. Add the sugar 1/4 cup at a time, then add the water and mix until blended. Lastly, add the flour and mix until it is just incorporated.
In a separate bowl, beat the egg whites with a whisk attachment at high speed until foamy, then add the cream of tartar and salt and continue beating until stiff peaks form, but do not over beat. Fold the egg white mixture into the bater until incorporated, then evenly distribute the batter between the three cake pans. Place in the oven and bake for 30-40 minutes or until the cakes turn golden brown on top. Turn the oven off and open the oven door halfway. Allow the cakes to cool in the oven for 30 minutes before running a knife around the edge of each pan to loosen the cakes grip. Flip the pans over onto a wire rack and remove the pan. Allow the cakes to cool completely, then trim the crust.
While the cakes are baking and cooling, you can prepare the whipping cream. Beat the heavy cream and 2 tablespoons of the watermelon syrup with a whisk at high speed until thick and fluffy, but take care not to over beat. Cover and refrigerate until use.
To assemble, place a layer of the cake on the serving surface. Spread a smooth layer of whipped cream on top of the cake, and then spread a layer of the curd on top of the whipped cream. Repeat until all of the toppings and cake layers are used. Serve immediately and refrigerate and leftovers.