2 chocolate bars, semi-sweet or milk chocolate*
2 1/2 tablespoons unsalted butter
2 large eggs, separated
1 teaspoon vanilla gel or extract
2 tablespoons confectioners' sugar
1 tablespoon Dutch-process cocoa, sifted
3 large egg whites
pinch of salt
Break up the chocolate into a large mixing bowl. Heat the cream and butter in a saucepan just to the boil. Pour the hot liquid over the chocolate, stirring to help dissolve the chocolate. When it is smooth, stir in the egg yolks one at a time and the vanilla.
Sift together the sugar and cocoa. Combine the two reserved whites with three additional ones in the work bowl of an electric mixer fitted with a whisk attachment or use a hand mixer. Begin beating the whites at a medium speed. Add a pinch of salt when the whites are foamy. Continue to beat until the whites are puffed but still soft. Sift on the sugar and cocoa mixture and continue beating until the whites are firm.
Stir one-quarter of the whites into the chocolate mixture. Gently fold in the remaining whites in three installments. You can eat this mousse now, but it will be better when it has chilled for an hour or two.
* If you use semi-sweet chocolate the mousse will be ready to eat immediately. When chilled, it will become firmer, like a brownie. The milk chocolate will be very soft after it's made. An hour or two in the refrigerator will give it more body.