Melt butter over medium heat, then add marshmallows and stir until smooth. Remove from heat and stir in Rice Krispies cereal. Reserve 3 cups of mixture for topping. Press the remaining mixture into a springform pan and chill in the refrigerator.
Whip chilled heavy cream until stiff peaks form; set aside. Beat cream cheese and confectioners’ sugar until smooth. Add Marshmallow Fluff and vanilla extract, mixing until silky. Gently fold whipped cream into the mixture.
Pour filling into the chilled crust and smooth the top. Use the reserved Rice Krispies mixture to form clusters and decorate the top.
Refrigerate for at least 4 hours or overnight. Release cheesecake from springform pan and slice carefully. Garnish with chocolate drizzle, whipped cream, or extra mini marshmallows if desired.
