1. Place about 1 or 2 inches of water in a large pot and bring to a gentle simmer. Combine the chocolate and espresso in a stainless steel or glass bowl and set the bowl over the mouth to the pot. Reduce the heat so the water under the bowl is just barely simmering, and melt, stirring occasionally, until the mixture is about 3/4 smooth. Remove the bowl from the heat and let the residual heat in the mixture finish the melting. Set the mixture aside to cool.
2. In a medium-sized bowl, whisk yolks together to break them up, then blend in the cooled chocolate mixture. Stir in the vanilla seeds and liqueur.
3. In a clean bowl with a clean, grease-free balloon whisk, beat the egg whites until stiff, but not dry and flaky. Fold the whites into the chocolate mixture in three stages, working quickly but using as few strokes as possible. Spoon the mixture into individual dessert glasses or bowls and refrigerate for 1 hour. Cover the dishes with plastic wrap and return them to the refrigerator for at least 3 hours, or overnight. Serve the mousse cool, garnished with a dollop of lightly sweetened whipped cream and a few chocolate-covered espresso beans, if desired.

Chocolate-Orange:

1. Add 2 teaspoons very finely grated orange zest to the egg yolks before adding the chocolate mixture. Use an orange-flavoured liqueur, such as Curacao or Grand Marnier in place of the coffee or chocolate liqueur. Garnish the mousse with lightly sweetened whipped cream and finely chopped candied orange peel.