Break the dark chocolate into small pieces (reserving 25g for grating later) and place in a bowl that fits snugly over a pan of boiling water. Leave until the chocolate has completely melted and then remove from the heat and set aside.
Add the crème fraiche to the melted chocolate and whisk until combined. Put the double cream and icing sugar into a separate bowl and whisk with an electric whisk until it forms soft peaks. Fold the chocolate mixture into the cream, add the coffee liquer and stir to combine.
Remove the chocolate covered honeycomb bars from the freezer, remove the packets and wrap them in a clean tea towel. Place the tea towel under a chopping board and press down hard on the chopping board to crush the honeycomb. Open up the tea towel and tip the crushed honeycomb into the mousse and gently fold through with a spatula. Spoon the mousse into four small serving dishes and then grate over the remaining chocolate and serve.