Melt butter in a medium saucepan over low heat. Add marshmallow fluff and stir until smooth. Remove from heat and stir in Rice Krispies until well coated. Press the mixture into the bottom of a greased 9-inch springform pan. Set aside.
In a large bowl, beat cream cheese with sugar and vanilla until smooth. Add eggs one at a time, beating well after each addition. Fold in sour cream. Pour the filling over the prepared crust.
Preheat the oven to 325°F (165°C). Bake the cheesecake for 45-50 minutes, or until set. Turn off the oven, leave the door slightly open, and allow the cheesecake to cool in the oven for 1 hour.
Sprinkle mini marshmallows and extra Rice Krispies over the top of the cooled cheesecake.
Drizzle with melted chocolate.
Refrigerate for at least 4 hours, or overnight.
