Preheat the oven to 350. Butter three 9-inch round cake pans and line the bottoms with parchment paper. Or these work great as cupcakes, put cupcake liners in muffin tins.
Sift the flour, baking powder and salt into a bowl and set aside.
In a separate bowl, stir together the egg whites and buttermilk and set aside as well.
In the bowl of a stand-mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy,about three minutes.
Beat in the vanilla extract. With the mixer running, add in 1/3 of the flour mixture, followed by 1/2 of the buttermilk mixture. Continue alternating the flour and milk, adding another 1/3 of the flour mixture, the remaining buttermilk, and then the rest of the flour.
Scrape down the sides of the bowl if necessary, and then beat the batter on medium for about a minute to make sure it's well incorporated. Turn off the mixer, and gently stir in the sprinkles.
Divide the batter between the prepared pans, and bake for about 30-35 minutes, or until the cake is golden, springy to the touch, and a tester inserted in the center of the cakes comes out clean. Or 20-22 minutes for cupcakes.