For the biscuit base: Melt butter in a small saucepan and stir in the crushed biscuits. Spoon mixture into a spring-clip 20cm (8 inch) tin. Spread evenly over the base of the tin.
Wipe rhubarb, cut into pieces and cook with 1 tablespoon water until it becomes soft; sieve and cool.
Pour 1 or 2 inches of cold water into a small pan; add the gelatine leaf and leave to soften for a few minutes. Then, over low heat, gently warm the gelatine until it begins to melt, stirring once or twice. Remove pan from the heat.
Sieve cottage cheese into a large bowl; beat in soured cream. Stir in rhubarb puree, caster sugar, egg yolks and softened gelatine.
Beat egg whites, add 1 tablespoon of the mixture, then lightly fold in remaining egg whites. Spoon on top of biscuit base and leave to set or chill in the fridge.
Loosen the sides of the cheesecake with a palette knife, then remove sides of tin and serve on the base.