Heat waffle iron. In medium bowl, mix egg yolks, milk, baking powder, flour, cocoa powder and melted butter. Mix til almost smooth. In bowl of mixer fitted with whisk attachment, whip egg whites til stiff peaks form, then add the brown sugar and continue whipping til stiff. Fold whipped egg whites into the batter. For each waffle, pour 1/2 to 1 cup of the batter (or the amount recommended by the waffle iron maker) onto the waffle iron; bake as directed by the manufacturer. Serves 3 to 4 or makes 3 to 4 large waffles.