Gently whip the chilled double cream.
Whisk 3 egg yolks with 50g sugar until pale and fluffy - sabayon.
Whisk the mascarpone in a bowl to loosen it and then add 1/3 of the sabayon.
Gently fold in the rest of the sabayon and the whipped cream.
Prepare the coffee and leave to cool. Add the cognac.
Dip the sponge fingers in the coffee for a few seconds.
Line the base of your serving dish with half of the sponge fingers and cover with a layer of the mascarpone mousse. Dust with cocoa powder.
Make a second layer of sponge fingers and cover again with the mascarpone mousse.
Top with cocoa powder or grated chocolate. Refrigerate for atleast 30 minutes to allow the mousse to set. Serve straight from the fridge.
«Be careful not to soak the sponge 'fingers' in the coffee for too long as they will become too soggy!»