Melt the chocolate in a bowl over a pan of simmering water (see
Whisk the egg whites in a clean grease-free bowl until softly peaking. Gradually whisk in the sugar, 1 tablespoon at a time, until the mixture is smooth and glossy.
Whip the cream in another bowl with the orange rind until softly peaking (take care not to overwhip or the cream will curdle). Fold in the orange liqueur.
Pour the melted chocolate into the cream mixture and mix together quickly. Beat in a spoonful of the egg white to loosen it, then carefully fold in the rest of the egg white. Mix together gently until all the egg white is incorporated.
Spoon the chocolate mousse into 10 espresso cups or small ramekins and chill for 1 hour. Serve each with an orange wedge.