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 (I measured out 1 heaping tablespoon for each). Place dough balls on greased cookie sheet then even flatten dough balls with your hand, to about 1/4" inch. Bake 9-10 minutes (9 1/2 was perfect for me). Cool completely, then frost.
Vanilla Frosting
In a mixing bowl, with an electric mixer, whip together butter and powdered sugar and cream or half and half until fluffy. Stir in vanilla. Add in food optional food coloring and mix until well blended.