Line the bottom of a springform pan (20 cm Ø) with baking paper. Melt butter. Separate 4 biscuits and remove the filling. Set aside separate biscuit halves for decoration. Put the rest of the filled biscuits into the large freezer bag.
Close the bag and run a dough roller over it until the biscuits are finely crumbled. Mix the 4 biscuits with the detached filling and liquid butter. Press to the bottom in the mould. Chill for about 10 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). For the filling, mix quark, 100 g condensed milk (put the remaining condensed milk in the refrigerator!) and egg with a whisk.
Spread on the biscuit base. Bake the cake in a hot oven for about 20 minutes. Remove and sprinkle with grated chocolate while still hot. Let the cake cool completely in the mould.
Whip the rest of the cold condensed milk and lemon juice until creamy with the whisk of the mixer, allowing the cream stiffener to trickle in. Spread the cream over the cake. Crumble the biscuits set aside and spread on the milk cream.
Chill for at least 4 hours, then remove from the mould and dust with cocoa to serve.