For the chocolate base, spread the bottom of a springform pan (26 cm Ø) with oil. Break the chocolate into pieces and melt in a hot water bath. Put the biscuits in the large freezer bag. Close the bag and run a dough roller over it until the cookies are finely crumbled.
Mix with the liquid chocolate. Press into the mould to form a flat base, forming a rim about 6 cm high. Chill for at least 2 hours.
For the cream, soak gelatine in cold water. Mix the crème fraîche and the quark. Dissolve gelatine at low heat. First stir 2 tablespoons of quark mixture into the gelatine, then stir the gelatine mixture into the remaining quark mixture.
Whip cream until stiff and fold in, halve cream. Stir 1 tbsp. caramel sauce into one half of the cream, stir the remaining caramel sauce into the rest of the cream. Spread the creams one after the other on the chocolate base.
Use a fork to twist in streaks. Chill the cake for at least 4 hours.
Carefully remove the cake from the tin and sprinkle with the roasted hazelnuts. Sprinkle with caramel syrup to serve.