2 1/2 cups of cake flour

3/4 cup all-purpose flour

1 tablespoon baking powder

1 teaspoon baking soda

3/4 teaspoon salt

1/2 cup (1 stick) unsalted butter, softened

1/2 cup vegetable shortening

1 3/4 cups sugar

1 tablespoon pure vanilla extract

1 large egg

1 1/2 cups ice cold water

3 large egg whites, at room temperature

1/4 teaspoon cream of tartar