Melt the chocolate in a bowl set over a saucepan of simmering water. Stir in the coffee, then gradually mix in the egg yolks one at a time. Leave to cool slightly.
Place the biscuits in a plastic bag and crush with a rolling pin to make fine crumbs. Melt the butter in a small saucepan then stir in the crumbs. Line 4 150 ml (¼ pint) ramekin dishes with clingfilm so that the film overhangs the edges of the dishes. Divide half the crumbs between the bases of the dishes and press flat.
Whisk the egg whites until soft peaks, then stir a spoonful into the chocolate mixture to loosen it slightly. Fold in the remaining egg whites using a metal spoon. Pour the mousse mixture into the dishes, level the surfaces then sprinkle with the remaining crumbs. Chill for 4–5 hours or until set.
When ready to serve, lift the puddings out of the dishes using the clingfilm and then peel the clingfilm away. Top each pudding with a spoonful of crème fraîche and then decorate with a light dusting of drinking chocolate.