For the mousse, melt the chocolate in a bowl set over a pan of simmering water, taking care that the bottom of the bowl does not touch the water. Remove from the heat and leave to cool for 2-3 minutes.
Beat the mascarpone with the coffee and icing sugar. In a clean bowl, whisk the egg whites until they form soft peaks.
Carefully stir the chocolate into the mascarpone mixture then beat in a quarter of the whisked egg whites.
Fold in the remaining egg whites, being careful not to knock out all of the air, and spoon the mixture into cappuccino cups. Place in the fridge for at least an hour to set.
For the cappuccino cream, whisk the double cream until it begins to thicken. Add the coffee and icing sugar and continue to whisk until the mixture holds soft peaks.
Carefully spoon the cream onto the mousse then return to fridge until ready to serve.
To serve, sprinkle the top of the mousse with a little ground coffee and serve with biscotti.