Preheat the oven to 350 F. Prepare three 8-inch cake pans by spraying the bottoms of the pans with non-stick baking spray, lining them with a circle of parchment paper, and then lightly spraying the paper as well.
In the bowl of your stand mixer fitted with the paddle attachment, beat the butter and sugar for 5 minutes on medium speed, scraping the bowl several times, until light and fluffy.
Beat in the eggs, one at a time, beating each for 15 seconds before adding the next.
Scrape the bowl down and beat in the vanilla.
In a separate bowl, sift together the flour, baking powder and salt. In another bowl, combine the buttermilk and sour cream.
With the mixer on low, add the flour mixture in three additions, alternating with the buttermilk/sour cream, starting and ending with the flour. Use a spatula to finish bringing the batter together so that you don't over-mix.
Divide the batter between the pans, smooth out the top of the batter and tap the pans a few times on the counter to release any bubbles.
Bake the cakes on the center oven rack for about 30 minutes, until a cake tester comes out clean.
Set the pans on a wire cooling rack, cover loosely with a clean kitchen towel, and cool completely in the pans before removing and frosting. It's normal for the cake to settle and shrink a bit as soon as you take it out of the oven.