- 1 cup unsalted butter, room temperature
- 2 cups granulated sugar
- 6 large egg whites
- 2 1/ 2 cups whole buttermilk
- 2 teaspoons vanilla bean paste or vanilla extract
- 3 1/ 4 cups cake flour, sifted
- 3 1/ 2 teaspoons baking powder
- 1 teaspoon coarse Kosher salt
- 1 1/ 2 cups unsalted butter, room temperature
- 3 cups powdered sugar
- 1 tablespoon meringue powder (optional)
- 1 1/ 2 teaspoons vanilla extract
- 1/ 8 teaspoon coarse Kosher salt
- 3 tablespoons milk or cream, if needed
Preheat the oven to 350 and grease three 8-inch round cake pans. Sift the flour together with the baking powder and salt and set aside. Make sure that your butter, egg whites and buttermilk are all at room temperature, for best results.
In the bowl of your stand mixer fitted with the paddle attachment, cream the butter and sugar for 10-12 minutes, scraping the bowl down every few minutes, until very light and fluffy and pale in color.
In a separate bowl, whisk together the egg whites, buttermilk and vanilla, just to combine. With the mixer on low, add the wet ingredients to the butter/sugar mixture and mix for 1 minute to combine; it will look lumpy/curdled. Add the flour, baking powder and salt to the mixture, mix on low for 30 seconds, and then beat on medium speed for 2 minutes for a light and airy batter.
Divide the batter between the cake pans. Bake for 25-30 minutes until the centers are done, then set the pans on a wire rack, cover loosely with a clean kitchen towel and cool completely.
In the bowl of your stand mixer fitted with the whisk attachment, beat the butter for 1 minute until smooth. With the mixer on low, add the powdered sugar, meringue powder and salt and mix until combined. Add the vanilla and milk, increase the speed to medium high and whip for 4-5 minutes, scraping the bowl occasionally, until very light and fluffy.
When the cakes are cool, run a knife around the edges, remove from the pans, and frost with the buttercream.