Sift flour, baking powder and cocoa. Fold into butter mixture alternately with milk.
In a clean bowl, whip egg whites until soft peaks form. Stir 3-4 tablespoons into batter then gently fold in remaining egg whites. Spoon into prepared pan.
Bake for 40-45 minutes, until a skewer inserted in centre comes out clean. Cool in pan for 5 minutes then turn out on to a wire rack.
To prepare the topping, beat butter and icing sugar, until creamy. Beat in melted chocolate. Stir well. Spread over cake.