Cream butter; gradually add sugar, beating until light and fluffy. Add eggs, one at a time, beating well after each addition. Add melted chocolate, and beat until blended.
Combine flour and baking powder; add to chocolate mixture alternately with bourbon, beginning and ending with flour mixture. Stir in vanilla and desired nuts. Pour batter into a greased 13- x 9- x 2-inch baking pan.
Bake at 350° for 25 minutes. Cool brownies completely. Spread with Chocolate-Bourbon Frosting. Cut into squares.