Preheat oven to 350 degrees. In a mixing bowl combine flour, baking powder, baking soda and salt, set aside. In the bowl of an electric mixer, whip together butter and sugar until pale and fluffy about 3-4 minutes. Stir in egg and egg white. Add sour cream and vanilla and mix until blended. Slowly add dry mixture and stir until well combine. Form dough into balls . Place dough balls on greased cookie sheet then even flatten dough balls with your hand, to about 1/4" inch. Bake 9-10 minutes . Cool completely, then frost.
