Sift together the flour, sugar, baking powder and soda. Set aside. Cream together the sour cream, butter, milk and vanilla; mix well. Stir in the flour mixture.
Drop by teaspoonfuls onto greased cookie sheets, flatten with fork. Sprinkle with sugar. Bake in the preheated oven for 10-12 minutes, until firm and golden coloured. Cool for 2 minutes in the oven and remove to a wire rack to cool completely.