To make prunes, add all ingredients, except brandy, to a small pot. Bring to a boil on a high heat, then reduce to a simmer and cover with a lid. Cook for about 8-10 minutes or until prunes are plump and liquid has almost disappeared. Remove from heat, pour over brandy and stir well. Allow to cool completely. Store prunes and syrup in an airtight container in the fridge, bringing them back to room temperature before serving.
For mousse, place chocolate, butter and cream in a microwave-proof bowl and heat in microwave in 30-second bursts, stirring after each, until just melted (about 2 minutes). Remove and stir well until the chocolate is smooth and glossy.
While chocolate is melting, whisk egg yolks in a bowl for about 1 minute until silky. Gently stir the yolks into the hot chocolate mixture. Stir in the espresso and orange zest.
Whip egg whites until soft peaks form, then add maple syrup and continue to whip until mixture forms a glossy, almost meringue-like consistency.
Take about a third of the egg whites and stir into the chocolate mixture until well incorporated. Then, as gently as possible, fold through the remaining egg whites. Be careful not to over-mix – stop as soon as there are no streaks remaining. Divide mousse between six to eight ramekins or small glasses and refrigerate for at least a few hours to chill.
To serve, spoon a couple of drunken prunes onto each chocolate mousse.