Grease a 9" (23 cm) round spring-release cake tin.
To make the base
Crush the crackers until they resemble fine breadcrumbs. Add the melted butter and crushed praline if using. Mix well and pour into your cake tin.Using the back of a spoon, press down hard until you have a smooth, solid cracker base. Refrigerate until cheesecake mixture is ready.
To make the cheesecake mixture
Purée of the raspberries in a blender, leaving some intact for decoration. Then press through a sieve. Reserve half of the purée; return the rest to the blender, add the yogurt, cream cheese, icing sugar and melted chocolate and blend until well mixed and evenly colored.
Soak the gelatin in a small bowl with a bit of cold water in a saucepan. Warm over a gentle heat, stirring, until fully dissolved. Do not let it cook!Add the gelatin to the cheesecake mixture and blend again until well incorporated. Pour the cheesecake mixture into your cake tin.
Spoon in the reserved purée and swirl with a knife to make a marbled pattern. Place the remaining raspberries on top, cover and chill for 3 - 4 hours.