Preheat the oven to 350 degrees F. This is crucial for ensuring your cake bakes evenly.
Add the sliced strawberries to one side of a parchment-lined baking sheet and the diced rhubarb to the other side. Sprinkle about 2 tablespoons of sugar onto the strawberries and the remaining 2 tablespoons onto the rhubarb. Toss them gently to coat and keep them separate, then roast them in the oven for about 10 minutes. This roasting step will concentrate their flavors.
After roasting, remove the sheet from the oven and let the strawberries cool for about 5 to 10 minutes. Once cooled, transfer the strawberries to a blender and pulse until smooth. You should end up with about 1/2 cup of puree. If you’d like, mix in 3 drops of red food coloring to enhance the color. Then, add the roasted rhubarb to the blender and pulse until it’s smooth as well. This will create a beautiful base for your frosting.
While your oven is still at 350 degrees F, grease a 13×9-inch baking pan and set it aside for later use.
In a medium bowl, combine the cake flour, baking powder, and salt. Whisk until combined and set the mixture aside.
In the bowl of a stand-up mixer fitted with a paddle attachment, add the unsalted butter and 1 3/4 cups of sugar. Beat them together until the mixture is light and fluffy. This should take about 2 to 3 minutes.
In a measuring cup, combine the milk, egg whites, vanilla extract, and the reserved 1/2 cup of strawberry puree. Whisk until combined. Gradually add this mixture to the butter and sugar while the mixer runs on low speed, alternating it with the dry ingredients. Start and end with the dry mix, ensuring the batter is relatively smooth without overmixing.
Pour the batter into the prepared baking pan. Bake in the preheated oven for about 25 minutes, or until a skewer inserted into the center comes out clean. Keep an eye on it to avoid overbaking!
Once baked, allow the cake to cool in the pan for about 5 minutes, then invert it onto a cooling rack. Let it cool completely to room temperature, which usually takes about an hour.
Meanwhile, prepare the frosting. In a saucepan, fill it with a few inches of water and place a clean bowl of your stand mixer atop it. Add 3/4 cup of sugar and 6 egg whites to the bowl. Mix them together and heat the mixture until it reaches 140 degrees F or until the sugar has completely dissolved.
Carefully transfer the bowl to the stand-up mixer and beat with the whisk attachment for about 5 to 7 minutes, until the mixture is very fluffy. Once fluffy, switch to the paddle attachment and, while the mixer is running on low speed, add in the unsalted butter one cube at a time until the mixture becomes smooth and glossy. Finally, pour in the rhubarb puree and the red food coloring, mixing until everything is well combined.
Once the cake has completely cooled, pour the frosting on top and use an offset spatula to smooth it out to the edges. Add some decorative swoops to the frosting to give it a personal touch, and don’t forget to top it off with a generous amount of sprinkles!
