Step 1: Preheat your oven to 350°F (175°C). Prepare your 9-inch round cake pans by greasing them with butter and dusting them with flour to prevent sticking.
Step 2: In a large mixing bowl, whisk together the cake flour (or all-purpose flour and cornstarch), baking powder, baking soda, and salt.
Step 3: In another bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy (about 3-5 minutes).
Step 4: Add the eggs and egg white, mixing until well combined. Gradually add buttermilk and vanilla extract, mixing until smooth.
Step 5: Slowly add the dry ingredients to the wet mixture, mixing just until combined. A few lumps are fine.
Step 6: Divide the batter evenly between prepared cake pans. Bake for 25-30 minutes or until a toothpick comes out clean. Cool in pans for 10 minutes before transferring to a cooling rack.
Step 7: For the frosting, combine sugar, buttermilk, unsalted butter, baking soda, and salt in a saucepan. Cook over medium heat, stirring frequently until boiling. Simmer for about 10-12 minutes until thickened.
Step 8: Remove from heat, stir in vanilla extract, and cool for about 30 minutes until spreadable.
Step 9: Once cakes are cool, place one layer on a serving plate, spread caramel frosting on top, place the second layer, and frost the entire cake.
