2 tablespoons rum, bourbon, Grand Marnier, or other liqueur (optional)
2 tablespoons very strong brewed coffee (espresso is best)
3 eggs, separated
1/2 teaspoon vanilla extract
2 tablespoons sugar
6 tablespoons cream, very cold
Instructions
Use a microwave or double boiler to melt the chocolate, butter, and liqueur if you’re using it; check and stir it frequently, then remove from the heat and stir until smooth. Stir in the coffee, and set the mixture aside until the bowl is cool enough to hold, then whisk in the egg yolks and vanilla.
In a medium bowl, beat the egg whites with 1 tablespoon of the sugar until they hold stiff peaks. In a separate bowl using clean beaters, beat the cream with the remaining 1 tablespoon sugar until it holds soft peaks.
Stir a couple of spoonfuls of the egg whites into the chocolate mixture to lighten it, then fold in the remaining whites, using thorough but gentle strokes. Fold in the cream until just combined, so that there are no streaks of white, then refrigerate until chilled. If you’re in a hurry, divide the mousse among 4 to 6 cups; it will chill much faster. Serve within a day.