Put the raisins and rum in a small saucepan. Warm over a gentle heat for a couple of minutes, then remove from the heat and stir it the orange zest. Leave the raisins to soak in the rum for 30 minute
Melt the chocolate in a bowl suspended over a pan of simmering water (make sure the base of the bowl doesn’t touch the water), the leave to cool a little; don’t skip this step, or you’ll make chocolate scrambled eggs instead of mousse! Stir the cooled, melted chocola into the egg yolks. Stir in the soaked raisins and some sugar, addir more or less depending on how much of a sweet tooth you have.
In a clean bowl, whisk the egg whites until they form stiff peaks. To help keep as much air in the soufflé as possible when folding the whisked egg whites into the chocolate mixture, first stir in just 1-2spoonfuls to loosen it up, then continue with the rest of the eg white, which should then fold in more easily.
Pour the mousse into a soufflé dish or individual serving dishes. Chill for at least 4 hours, or overnight. Serve with single cream poured over the top.