Strawberry Marshmallow Sheet Cake

Ingredients

• 2 ¾ cups unbleached cake flour
• 2 tsp. baking powder
• ¾ tsp. kosher salt
• ½ tsp. baking soda
• 1 lb. very ripe fresh strawberries, hulled, divided
• 1 ½ cups granulated sugar
• ¾ cup unsalted butter, softened
• 1 large egg, at room temperature
• 2 large egg whites, at room temperature
• ⅓ cup whole buttermilk
• 1 ¾ tsp. vanilla extract
• 2 Tbsp. freeze-dried strawberries (from 1 [0.8-oz.] pkg.)
• ¾ cup butter
• 1 (7-oz.) container marshmallow creme
• 2 cups powdered sugar
• 1-2 Tbsp. whole milk

Instructions

1. 1Preheat the oven to 350°F. Grease a 13-by-9-inch baking pan with baking spray and line the bottom with parchment paper so the cake releases easily. In a medium bowl, whisk the cake flour, baking powder, kosher salt, and baking soda until well combined.
2. 2Put 1 cup of the fresh strawberries into a food processor and pulse 3 to 5 times until roughly chopped, then set them aside. Process the remaining strawberries for about 30 seconds until they form a smooth puree, yielding roughly 3/4 cup. Set the puree aside.
3. 3In a stand mixer fitted with a paddle attachment, beat the granulated sugar and softened butter on medium-high speed for about 3 minutes until pale and fluffy. Add the whole egg, followed by the egg whites one at a time, mixing on medium speed just until blended. Mix in the buttermilk, vanilla extract, chopped strawberries, and strawberry puree for about 30 seconds until combined.
4. 4Turn the mixer to low speed and gradually add the dry flour mixture in three parts. Mix just until incorporated to avoid a tough cake, scraping down the sides of the bowl as necessary.
5. 5Pour the batter into the prepared baking pan and spread it into an even layer. Bake for 30 to 35 minutes, or until the edges turn golden and a wooden pick inserted into the center comes out clean. Move the pan to a wire rack and let the cake cool completely for about 1 hour and 30 minutes.
6. 6Grind the freeze-dried strawberries in a clean food processor for 1 minute until they become a fine powder, then set aside. In a stand mixer, beat the butter on medium-high speed for 2 minutes until creamy. Add the marshmallow creme and beat for another 2 minutes.
7. 7Turn the speed to low, mix in 1 cup of powdered sugar, then add 1 tablespoon of milk, the strawberry powder, and the remaining 1 cup of powdered sugar. Beat until completely smooth, adding a second tablespoon of milk if the frosting feels too stiff, and chill until needed.
8. 8Spread the chilled berry-marshmallow frosting evenly over the top of the cooled cake, then slice into squares.

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