Directions

1. Preheat oven to 325°F. Butter or oil baking pan, line bottom of pan with parchment paper; butter paper and dust baking pan with flour, tapping out any excess. Set aside.
2. Sift together flour, baking powder, baking soda, and salt into a medium bowl, and set aside.
3. Combine buttermilk and vanilla in another bowl, and set aside.
4. Beat butter, oil, and sugar in a bowl of an electric mixer fitted with the paddle attachment and cream until pale and fluffy, about 3 minutes.
5. Reduce speed and add egg whites one at a time. Scraping down mixer bowl between each addition. Add egg yolk and mix until combined.
6. Reduce speed to low. Add dry ingredients in three batches, alternating with the buttermilk mixture. Starting and ending with the dry ingredients. Be careful not to over mix the batter.
7. Pour batter into prepared pan, and smooth top with a spatula. Bake at 325 degrees until a toothpick inserted near the center comes out clean and the cake springs back when pressed lightly in the center, about 30-40 minutes.
8. Transfer to a wire rack to cool before unmolding.
9. To unmold, run a knife around the edge of the cake to loosen cake sides, invert cake to release from pan. Cool completely before decorating.