Ingredients

• Nonstick spray
• 5 cups all-purpose flour
• 4 cups cake flour
• 4 teaspoons baking powder
• 2 teaspoons baking soda
• 2 teaspoons fine salt
• 16 large egg whites, lightly beaten
• 4 cups sour cream
• 2 cups milk
• 6 tablespoons clear vanilla extract
• Seeds from 2 vanilla beans
• 1 pound unsalted butter, softened
• 5 1/3 cups granulated sugar
• 1 pound fresh strawberries
• 1/2 cup granulated sugar
• 1/3 cup balsamic vinegar
• 2 tablespoons freshly-squeezed lemon juice
• 1 cup mascarpone cheese, room temperature
• 2 cups heavy cream
• 1/2 cup confectioners' sugar
• 1 tablespoon clear vanilla extract
• 1 pound unsalted butter, softened
• 2 cups shortening
• 2 pounds confectioners' sugar, sifted
• 4 teaspoons clear vanilla extract

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Instructions

1. Special equipment: parchment paper; three 8-inch round cake pans
2. Preheat the oven to 325 degrees F. Spray three 8-inch round pans with nonstick spray and line them with parchment paper. Spray the paper.
3. Sift together the all-purpose flour, cake flour, baking powder, baking soda and salt in a large bowl. Whisk together the egg whites, sour cream, milk, vanilla and vanilla bean seeds in another large bowl.
4. Beat the butter and sugar with a paddle attachment until light and fluffy. Alternate beating in the flour mixture and the egg mixture, adding the flour mixture in 3 additions and the egg mixture in 2, starting and ending with the flour.
5. Fill each cake pan with 2 1/2 cups of batter and bake until a toothpick inserted in the center comes out clean, about 40 minutes. Cool the cakes in the pans for 15 minutes, then flip them onto a cooling rack to cool completely.
6. For the filling: Put the strawberries, granulated sugar, balsamic vinegar and lemon juice in a medium saucepan over medium heat, bring the mixture to a simmer, and cook until it gets to a syrupy reduction, about 30 minutes.
7. Cool completely.
8. Beat the mascarpone in a medium bowl until smooth. Whip the heavy cream until light and fluffy in another bowl; just before it is finished, whip in the confectioners¿ sugar and vanillia extract. Fold in the strawberry mixture.
9. For the frosting: Cream the butter and shortening until light and fluffy, then beat in the confectioners' sugar and vanilla extract.
10. To assemble the cake: Put 1 cake layer on a cake board or platter. Spread half of the filling mixture on top. Repeat with another cake layer. Frost the cake with the buttercream frosting and serve.