Put all but one biscuit in a freezer bag and crumble it with the dough roll. Melt the butter. Mix butter and crumbs and put them on the oiled bottom of a springform pan (22 cm Ø) and press them down.
Place the mould in a cool place for about 30 minutes.
Roughly chop the chocolate. Heat 200 g cream and remove from the heat. Melt the chocolate in it and let it cool down while stirring occasionally.
Whip the rest of the whipped cream until stiff, pour in the cream firming agent and carefully fold it into the chocolate cream. Spread the cream on the biscuit base and smooth it down. Chill the cake overnight.
Remove the cake from the mould before serving. Crumble 1 biscuit and sprinkle it on the cake.