Instructions: Preheat oven to 350 degrees F
Sift together flour, baking soda and cream of tartar; set aside. Cream together butter, sugar, and brown sugar in a bowl until light and fluffy, using an electric mixer at medium speed. Add eggs one at a time, beating well after adding each one. Add dry ingredients alternately with sour cream to creamed mixture, beating well after each addition using an electric mixer at low speed.

Drop cookie batter by rounded teaspoonfuls, about 3" apart on greased baking sheets. Bake for 8-10 minutes, or until edges are browned. Remove from baking sheets to cool on racks. Frost with vanilla frosting. Vanilla Frosting: Combine 4 cups of sifted confectioners sugar, 1 tsp. vanilla, and 4 TBSP of milk in a bowl. Beat until smooth. Add 1 more Tbsp of milk, if necessary, to make frosting easier to spread.

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