Remove the stems and hulls from about 1 pound 4 ounces of strawberries, and blend in a food processor until pureed. Transfer the puree to a small saucepan and heat over medium heat. Cook for 15-20 minutes, stirring frequently until reduced by about half so you have 1 1/4 cups. Allow to cool completely and refrigerate before using it in the cake. You will use 1 cup for the cake batter and the remaining 1/4 cup for the frosting.
1 cup fresh strawberry puree
Preheat oven to 350°F. Line two 9-inch or three 8-inch pans with parchment paper circles.
Beat butter and sugar in a large bowl until creamy and light. Add egg whites and vanilla extract. Add 1 cup of reduced strawberry puree, reserving the remaining 1/4 cup for the frosting. Add flour, baking powder, baking soda, and salt alternately with the buttermilk. Stir in the chopped fresh strawberries.
1 cup salted butter, 2 cups granulated sugar, 6 large egg whites, 2 teaspoons vanilla extract, 1 cup buttermilk, 3 cups cake flour, 3 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon table salt, 1 cup finely chopped strawberries
Divide between pans and bake 40-44 minutes for two 8-inch cakes, 38-40 minutes for two 9-inch cakes, 32-36 minutes for three 8-inch cakes.
By the end of the baking time, you should be able to insert a toothpick into the thickest part of each cake layer and it comes out clean.
Let the cake cool in their cake pans for a few minutes before turning them out onto wire racks to cool completely before assembling the full cake.