1. Preheat the oven to 190°C/375°F/Gas Mark 5. Grease and line an 20cm/8in spring clip cake tin. Place the biscuits in a plastic bag and crush with a rolling pin. In a bowl lightly beat 1 egg white, add the biscuits and mix. Press evenly over the base of the cake tin and bake for 15 minutes until firm and golden. Leave to cool.
2. Dissolve the gelatine in 4 tablespoons of boiling water. Mix the quark with the fromage frais, liqueur and sweetener. Whisk the remaining egg whites until stiff, then fold into the quark mixture with the gelatine. Pile on top of the base and smooth the top. Chill for at least 3 hours until set.
3. Release the cheesecake, slice and transfer to individual serving plates. In a pan gently heat the sugar with 1 tablespoon water, stirring, until melted. Raise the heat and boil without stirring for 3-4 minutes until golden. Drizzle over each slice and serve immediately.
Tip: You can make the cheesecake a day in advance - simply cover it in the tin and chill. Add the caramel just before serving.
Agar agar can be used instead of gelatine if preferred.
The government advises that because of a very slight risk of salmonella, vulnerable people such as the elderly, babies and pregnant women should not eat raw eggs.