Melt the chocolate in a bowl placed in another bowl of warm water on the stove. Whip the egg whites and begin to add 1 tbsp of the powdered sugar at a time, stirring nonstop.
Mix the cream, orange rind and orange liqueur, then add the melted chocolate to them.
Carefully add the beaten egg whites as well. Distribute the mousse into 10 espresso glasses and cool for 1 hour. Serve the mousse with slices of orange.