Preheat the oven 375°.
Butter an 8 by 8 inch square pan
In an electric mixer, combine the eggs and superfine sugar. Beat until light and fluffy.
In a medium saucepan, over medium heat, melt the bittersweet chocolate, milk chocolate and butter, stirring until smooth. Remove from heat and beat into the egg mixture.
Sift the flour and cocoa together and beat into the chocolate mixture. Whisk in the Guinness.
Pour into prepared pan and bake for 20 to 25 minutes, or until skewer inserted in the center comes out almost clean. Remove from the oven and let cool on a wire rack. To serve, dust the brownies with confectioners' sugar and cut into squares.
