Butter the sides of a 9" round cake pan (or spray with non-stick cooking spray).
Preheat oven to 350 degrees F
With a stand or hand mixer, beat together the butter and sugar until creamy. Beat in the vanilla extract. Add the egg yolks one at a time. Add the flour mixture and milk alternatively in small amounts (3 or 4 batches), mixing thoroughly and scraping down the sides of the bowl after each addition.
In a clean bowl, with clean mixing blades, beat the egg whites with the cream of tartar just until the egg whites hold a firm peak. Gently fold 1/2 of the egg whites into the cake batter until fully incorporated. Repeat with the remaining egg whites. Pour the batter into the cake pan. Bake for 45-55 minutes or until the cake has browned and a toothpick inserted in the center of cake comes out clean. Cool on wire rack for 10 minutes. Run knife around edge of pan to make sure cake removes cleanly. Place serving plate on top of cake pan, invert and remove cake pan.