Spread the bottom of a springform pan (26 cm Ø) with oil. Put the biscuits in a freezer bag, close the bag and crumble the biscuits finely with a dough roller. Melt butter in a saucepan, mix with cookie crumbs and 1 tbsp. sugar.
Put the crumbs into the mould and press them down to an even base. Chill for about 30 minutes.
Whip cream cheese, 100 g sugar, 1 pinch of salt and vanilla sugar with the whisks of the mixer for 3-4 minutes until creamy. Briefly stir in the flour and 100 ml egg liqueur. Stir in the eggs one after the other. Finally stir in mascarpone.
Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer). Seal the springform pan all around well with foil so that nothing can leak. Spread the cheese mixture on the dough and smooth it down.
Place the springform pan on the fat pan of the oven. Pour on boiling water so that the form for 1⁄3 is in the water. Bake in the hot oven for 60-65 minutes.
Take the cake out of the oven and remove the foil. Leave to cool on a cake rack for about 2 hours. Remove the cake from the mould. Whip cream until stiff and spread loosely on the cake. Chill the cheesecake for about 30 minutes.
Cut into pieces and shortly before serving, drizzle about 1 tbsp (approx. 20 g) of advocaat on each piece of cake.