Beat egg whites, cream of tartar and sugar.
In a separate mixer bowl, cream butter and sugar. Add egg yolk and vanilla.
In a separate bowl, whisk together the flour, salt, and baking powder. Add this dry mixture, alternating with the milk, to the butter mixture on medium speed until barely combined.
Using a spatula, fold 1/4 of the egg whites into the batter. When completed, add the remaining egg whites until just combined.
Pour into an 8 inch round cake pan or 18 muffin cups and bake at 350 until a toothpick comes out clean. For the cake, that's about 45 minutes; for the cupcakes, about 20 minutes.
Variations: Lemon Cake: add 2 t. lemon zest to butter mixture.
Raspberry/Apricot Swirl: Swirl in 3 T. raspberry / apricot preserves after pouring half the batter into the cake pan.
Frost with Basic Buttercream, page.