Prepare half of the sugar with the water for about 2 minutes.
Remove from the oven, finely crush with the chestnuts.
Melt the chocolate in the whipping cream and butter. Beat the yolks with the remaining sugar and vanilla until creamy and add the liquid chocolate-cream mixture while stirring.
Then fold this mixture into the pureed chestnuts form. Season with cognac. Whip egg whites to egg whites and fold into the mixture.
Fill into a glass bowl, or possibly into portion glasses, cool.