Gently melt the chocolate in the microwave or a bowl set over a saucepan of simmering water then cool slightly.
Beat together the egg yolks and Cointreau and gently mix into the melted chocolate. Whisk the egg whites to firm peaks and carefully fold them through the chocolate mixture. Spoon into 4 glasses and chill.
To make the candied orange, dissolve the sugar in 2 tbsp water and boil for 2 minutes. Drop in the orange zest and cook for a couple of minutes, then remove, drain and leave to cool on baking paper. Spoon some cream on top of the mousse and top with the candied orange.