In a large pot over medium-low heat, melt the butter completely. Add mini marshmallows and stir until smooth and glossy, about 4–5 minutes.
Remove from heat and gently fold in Rice Krispies until evenly coated. Press two-thirds of the mixture into the bottom and 1 inch up the sides of a 9-inch springform pan. Reserve the rest for the topping. Let the crust cool completely.
In a chilled bowl, beat the heavy cream until stiff peaks form. Set aside.
In another large bowl, beat cream cheese until smooth (about 2 minutes). Add confectioners’ sugar, Marshmallow Fluff, and vanilla extract. Beat again until creamy and lump-free.
Gently fold whipped cream into the cheesecake mixture until no white streaks remain. Pour into the prepared crust and spread evenly.
Break the reserved Rice Krispie mixture into clusters and scatter over the cheesecake edges.
Refrigerate for at least 4 hours or overnight until fully set. Carefully release the springform pan before slicing.
