For the caramel cheesecake, place the biscuits in a freezer bag and smash to crumbs with a meat mallet.
Melt the chocolate with the butter in a water bath. Mix the crumbs, nuts, chestnut puree and chocolate mixture together.
Line the bottom of a springform pan (20-22 cm) with baking paper, pour in the base mixture and press down well.
Beat the eggs with sugar and vanilla sugar until foamy. Stir in 200 g dulche de leche, the curd cheese, crème fraiche, the cream, sieve the custard powder over it and also mix well.
Finally, fold in the mascarpone (do not stir too long, otherwise the mascarpone will liquefy too much).
Spread the mixture on the prepared base and bake in the oven at 160°C for about 60 minutes. Open the oven door and let the cake cool slowly.
When the cake has cooled, spread the remaining dulche de leche on top and sprinkle with nut brittle.
Refrigerate for a few hours. Remove the caramel cheesecake from the refrigerator in time for the flavor to develop well before serving.