Preheat oven to 180°C.
Combine flour, baking powder and salt in medium bowl; set aside. Melt butter in large saucepan over low heat; remove from heat. Stir in cocoa. Beat in brown sugar, sugar, eggs and vanilla. Stir in dry ingredients and walnuts. Spray or butter a 23 cm square pan. Spread batter in pan.
Bake in preheated 180°C oven for 40 minutes. Do not overbake. Cool completely.
Melt butter over low heat in medium saucepan. Remove from heat. Stir in cocoa and vanilla. Stir in icing sugar and milk until smooth and of spreading consistency. Spread icing over cooled brownies. Decorate with sprinkles.
