1 cup (2 sticks) unsalted butter, room temperature, plus more for pans
2 1/2 cups all-purpose flour (spooned and leveled), plus more for pans
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1 teaspoon salt
1 1/2 cups sugar
2 large eggs
1. Preheat oven to 350 degrees. Butter and flour two 8-by-2-inch cake pans, tapping out excess flour. In a medium bowl, whisk flour, baking powder, baking soda, and salt.
2. In a large bowl, using an electric mixer, beat butter and sugar until light and fluffy. With mixer on low, beat in eggs and yolks, one at a time. Beat in vanilla.
3. Alternately beat in flour mixture and buttermilk, beginning and ending with flour mixture; mix just until combined.
4. Divide batter between pans; smooth tops. Bake until cakes pull away from sides of pans, 32 to 35 minutes. Let cool in pans 10 minutes. Run a knife around edges of pans and invert cakes onto a wire rack. Let cool completely.
5.Place one cake, bottom side up, on a cake stand. Tuck strips of parchment paper underneath. Using an offset-spatula or table knife, spread top with Whipped Frosting. 6. Top with remaining cake; frost top, then sides.