In saucepan, melt butter and brown sugar. Transfer to mixing bowl. Beat in eggs, one at a time. Mix in vanilla and 1/4 c. liqueur. Combine flour, cocoa and baking powder; stir into batter mixture until smooth. Add walnuts. Pour into greased and floured 9 x 13 baking dish.
Bake at 350 degrees just until sides begin to shrink from pan. approx. 25 to 35 minutes. Do not over bake. Remove from oven and burs top with 2 tbsp liqueur.
Frosting: Cream 3 Tbsp butter. Add 1/2 pound powdered sugar and 3 tbsp cocoa. Beat in 3 tbsp creme de cacao liqueur. With mixer running, add 2 tbsp hot coffee. Whip until thick and smooth, using additional sugar or corree to correct consistency
The frosting is optional since these brownies are rich and good without it.