Place the chocolate in a bowl and melt either over a pan of hot water or in a microwave for 30 seconds. Stir in the melted butter.
Whisk the egg yolks and sugar together until light and thick (the ribbon stage), then pour into a large bowl.
Whisk the egg whites until stiff, and then leave to one side.
Take four large wine glasses and place three of the biscuits in the bottom of each. Pour half the wine evenly into the bottoms of the glasses.
Stir the orange zest and juice into the sugar and egg mixture and then quickly fold in the melted chocolate followed by the egg white. Be careful and take your time with this stage so as not to lose the air.
Half fill the glasses with this mousse mixture and then top with the remaining biscuits.
Pour the rest of the wine over the top and then fill the glasses up to the top with the remaining mousse.
Put in the fridge to set for a few hours, preferably overnight, before serving.
