Blend together 1/2 cup butter, brown sugar, egg, vanilla, and sour cream. Add in the baking powder, soda, and flour. Stir in the nuts.
Drop dough by teaspoonfuls onto buttered baking sheets. Cook at 375 degrees F (190 degrees C) for 8 to 10 minutes. Remove from oven, and cool on wire racks.
To Make the Icing: Heat 1/2 cup butter or margarine in a saucepan until it is light amber colored. Remove pan from heat, and stir in confectioners' sugar and the milk. I sometimes have to use more milk to get the icing smooth and creamy. Frost cookies.