1) If desired, rim the glass with chocolate: pour some ground chocolate onto a small plate. Roll the outer top edge of the champagne flute in the chocolate.
2) Pour the stout into the glass, taking care to make as little foam as possible. (The best way to do this: tilt the glass and pour the beer slowly down the inside of the glass.)
3) Slowly pour Prosecco down over the back of a spoon and into the glass, layering the two alcohols for a dramatic cocktail. Drink.