Step 1

Preheat the oven to 350 degrees F. 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 sprinkle the remaining 2 tablespoons of sugar onto the rhubarb. Toss them gently and keeping them separate, transfer them to the oven to roast for 10 minutes.

Step 2

Remove and allow them to cool for about 5 to 10 minutes. Add the strawberries to a blender and pulse until smooth. (You should end up with about 1/2 cup puree. Mix in 3 drops of food coloring, if you like.) Next, add the rhubarb to the blender and pulse until smooth.

Step 3

Keep the oven at 350 degrees F (as it was for roasting the strawberries). And grease a 13x9-inch baking pan. Set aside.

Step 4

In a medium bowl, add the cake flour, baking powder and salt. Whisk until combined. Set aside.

Step 5

To the bowl of a stand-up mixer with the paddle attachment, add the butter and sugar. Beat until light and fluffy, about 2 to 3 minutes. In a measuring cup, pour in the milk, egg whites, vanilla extract or paste and reserved 1/2 cup strawberry puree (discard any extra) and whisk until combined.

Step 6

Turn the mixer to low speed and, alternating between the dry ingredients and the liquid ingredients (the strawberry/egg white mixture), add the them butter/sugar mixture until the batter is relatively smooth.

Step 7

Pour the batter into the prepared baking pan and transfer to the oven to bake for 25 minutes, until a skewer inserted into the center comes out clean.

Step 8

Allow the cake to cool in the pan for about 5 minutes and then invert onto a cooling rack and cool to room temperature. It’ll probably take a good hour.

Step 9

Bring a saucepan filled with a few inches of water. Place the clean bowl of a stand-up mixer atop the saucepan. Add the sugar and egg whites to the bowl and mix.

Step 10

Heat the mixture until it reaches 140 degrees F or until the sugar has dissolved completely. Carefully (it’ll be hot!) transfer the bowl to the stand-up mixer and with the whisk attachment, beat for 5 to 7 minutes, until very fluffy. Remove the whisk attachment and replace with the paddle attachment. With the mixer running on low speed, add the butter, one cube at a time until smooth and glossy. Lastly, pour in the rhubarb puree and red food coloring and mix until combined.

Step 11

When the cake is completely cool, pour the frosting on top of the cake. Using an offset spatula or any spatula you own, smooth it out until it reaches the edges. Make a few swoops in the frosting so it looks cute. And then top it with a bunch of sprinkles!