Preheat oven to 350 degrees F.
Prepare 2 x 8 or 9 inch cake pans by lining the bottom with parchment paper and greasing the sides with butter or vegetable shortening.
Sift together flour and baking powder, set aside.
With a stand or electric mixer fitted with a paddle attachment, cream butter and sugar together until light and fluffy, at least 3 minutes.
Alternate adding in the flour/baking powder mixture, beat until ingredients are just combined. Stop mixer, scrap down the sides of the bowl with a spatula, and finish mixing batter by hand so as not to overmix.
In a separate bowl, beat egg whites until stiff peaks form, 4-10 minutes.
Fold egg whites into batter by hand, taking care not to knock out the air.
Bake at 350 degrees F for 25-30 minutes.
Allow cake to cool completely, then top with your favorite frosting.
I used American buttercream for this cake.