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.
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.
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.
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.
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.
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.
For the frosting: Cream the butter and shortening until light and fluffy, then beat in the confectioners' sugar and vanilla extract.
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.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.