Preheat the oven to 350 F, and spray the bottoms of three 8-inch cake pans with non-stick spray.
In the bowl of your stand mixer fitted with the paddle attachment, beat the butter, sugar and vanilla bean on medium speed for 10 minutes, scraping the bowl occasionally.
Beat in the egg whites, one at a time, beating each for 10 seconds before adding the next. Scrape the bowl down and beat for 10 more seconds.
In a separate bowl, sift together the flour, baking powder and salt.
With the mixer on low, add the flour mixture in three additions, alternating with the buttermilk, starting and ending with the flour. Use a spatula to scrape the bowl well and incorporate any stray bits of flour.
Divide the batter between the pans. Bake on the center oven rack for about 25-30 minutes, until a cake tester or toothpick comes out clean.
Set the pans on a wire rack, cover loosely with a clean kitchen towel, and cool completely before frosting.