Melt the chocolate and liqueur together in a 3-quart heavy-bottomed saucepan over low heat, stirring to blend. Remove from heat and let cool for 5 minutes.
While the chocolate is melting and cooling, separate the eggs and whip the egg whites until stiff, but not dry; whip the cream until quite stiff.
Add the egg yolks to the chocolate mixture all at once and whisk briskly until fully combined.
Fold the beaten egg whites into the chocolate and egg mixture half at a time, then fold in the whipped cream.
Transfer the mousse to individual serving dishes and chill until firm, at least 3 hours.
Garnish with whipped cream and shaved chocolate or raspberries.