Blend together flour, baking powder and salt and set aside. Melt butter over low heat in a large saucepan and remove from heat. Stir in cocoa then beat in brown sugar, white sugar, eggs and vanilla. Stir in dry ingredients and walnuts.
Spray a 9 inch square pan (23 cm) with cooking spray and spread batter in pan. Bake in a 350°F (180°C) oven for 40 minutes. Do not over bake. Cool completely. Spread Fudgey Icing (recipe above) over top.
Prepare Fudgey Icing
Melt butter over low heat in a medium saucepan and remove from heat. Stir in cocoa and vanilla. Stir in icing sugar and milk until smooth and of spreading consistency. Cool for 2 minutes before spreading.