Cream the butter and brown sugar in a mixing bowl until light and fluffy.
Beat in the egg and vanilla.
Mix together the flour, baking powder, baking soda and salt.
Add to the creamed mixture alternately with the sour cream, mixing well after each addition.
Fold in the cashews carefully.
Drop by teaspoonfuls 2 inches apart onto greased cookie sheets.
Bake at 375 degrees for 10 minutes.
Remove to a wire rack to cool.
Frost with golden butter frosting.