Melt the butter and let it cool. Roughly crumble the rusks and place them in a sturdy bag. Roll the rolling pin back and forth until everything is very finely crumbled.
Mix the butter with the rusk crumbs, cinnamon and vanilla sugar and press the bottom into the ungreased springform pan.
Preheat the oven to 150 degrees (convection without preheating: 140 degrees).
Beat the eggs and sugar with the whisk until frothy. Add the cream cheese spoon by spoon and knock in well. Add lemon juice. Now fill the dough into the mold, distribute it well and press it down.
Bake the cake in the oven on the middle rack for about 1 hour. Mix the crème Fraiche with the vanilla sugar and spread on the cheese topping. Bake for another 10 minutes.