Break 125 g chocolate into pieces and put them into a small pot together with the butter. Melt while stirring over low heat (or on a hot water bath). Let it cool down a little. Separate the eggs. Stir egg yolks and icing sugar briefly, then add the cognac and finally the liquid chocolate.
Beat the egg whites with the whisks of the hand mixer until stiff and carefully stir/pull into the chocolate cream. Divide the chocolate cream into 4 glasses and chill for 1-2 hours. Whip cream until semi-stiff (do not sweeten!). Use a vegetable or parmesan slicer to slice shavings off the dark chocolate coating. Shortly before serving, pour a dollop of cream on each mousse and sprinkle with chocolate shavings