For the cookies
Blend together butter and brown sugar. Add eggs, one at a time; blend well. Add vanilla; beat until light.
In a separate bowl, stir together flour, baking powder, baking soda and salt. Add flour mixture to butter mixture alternately with sour cream; mix well.
Drop by rounded tablespoonfuls onto non-stick or lightly greased baking sheets. Bake at 350 degrees for 8 minutes.
Remove to a wire rack; cool.
Frost with Browned Butter Frosting.
For the Browned Butter Frosting
Melt butter in a saucepan over medium heat until it turns golden. Watch carefully as this will happen quickly. Remove from heat. Stir in powdered sugar and enough milk to make a spreading consistency.