White Vanilla Bean Cake with Balsamic Strawberry and Mascarpone Filling

Cake:

• 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 

Balsamic Strawberries and Mascarpone Filling:

• 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

Buttercream Frosting:

• 1 pound unsalted butter, softened
• 2 cups shortening 
• 2 pounds confectioners' sugar, sifted
• 4 teaspoons clear vanilla extract