This is most easily done with an electric mixer, although you can use a heavy wire whip. Spread the frosting over the brownies. Melt the remaining 3 oz chocolate, and spread over the frosting just put on the brownies. Cover as evenly as possible. Cool for several hours in a refrigerator, then bring to room temperature and cut into pieces to serve.
Notes Be very careful when melting chocolate.
It burns easily. A double boiler is probably the best way thing to use. It is critical to cook the first frosting to just the right point. If cooked too little, it will be too thin to beat to a creamy consistency. If cooked too much, it will be too brittle.