Preheat Your Oven: Start by preheating your oven to 180°C (350°F). Grease a springform pan to ensure an easy release later.
Prepare the Biscuit Base: Crush the butter biscuits into fine crumbs. Melt the 90 g of butter, and mix it with the crumbs until combined. Press the mixture firmly into the bottom of the springform pan.
Melt the Chocolate: While the base sets, melt the milk chocolate over a double boiler or in the microwave, then set aside to cool slightly.
Mix the Creamy Filling: In a large mixing bowl, combine the low-fat quark, sour cream, sugar, and vanilla sugar. Mix until the mixture is smooth and creamy.
Add Chocolate and Eggs: Stir the melted chocolate into the creamy filling, and then add the cornstarch and eggs—mixing well after each addition until everything is fully incorporated.
Bake: Pour the cheesecake mixture over your prepared biscuit base, spreading it evenly. Bake for 50-60 minutes, or until the center is set.
Cool and Chill: Once baked, let the cheesecake cool to room temperature, then refrigerate it for at least 4 hours (or overnight) to develop its flavors completely.