Line and grease a 20 cm springform pan with baking paper. Place the gelatin sheets in a bowl with cold water.
Melt the butter in the pan. Place the cookies in a bag and crumble them with a rolling pin. Mix the cookie crumbs with the butter. Spread the cookies on the bottom of the pan and press down well.
Whip the double cream (save a few tablespoons) until soft peaks form. Beat the monchou with the salt and mix well with the whipped cream. Gently heat the remaining tablespoons of the double cream and melt the gelatin in it. Mix the gelatin through. Then mix the dulce de leche through the cream cheese mixture. It is nice not to mix it too well so you can still see some caramel lumps through it.
Pour everything into the springform pan. Let set in the refrigerator for two hours.
Break the chocolate into small pieces. Heat the cream in a saucepan until it starts to boil, remove from the heat and add the chocolate.
Stir well and when everything is mixed, stir in the cold butter, which will give it a fine shine. Spread the mixture over the cake from the center and let it drip off carefully. Decorate with the ferrero rocher.