In a small mixing bowl, combine flour, baking powder, baking soda, and salt, and set aside
In a larger mixing bowl, cream butter and sugar together until light
Beat in sour cream, egg, and vanilla
Stir in dry ingredients until just combined
Drop by the tablespoon onto an ungreased, or parchment-lined baking sheet, leaving about 2 inches of space between each cookie to account for spread, and heat distribution
Bake in center of oven for 10 minutes
Remove from oven, and transfer cookies to a wire rack to cool
When cookies are cool, beat frosting ingredients together until smooth, and spread over each cookie.