Preheat your oven to 325°F (163°C). Grease the bottom of a 9-inch springform pan.
In a large saucepan over low heat, melt the butter. Once melted, add the mini marshmallows and stir until completely melted and smooth. Remove from heat.
Stir in the 3 cups of Rice Krispies cereal until well coated. Press the mixture firmly into the bottom of the prepared springform pan to form an even crust. Set aside to cool.
In a mixing bowl, beat the softened cream cheese with an electric mixer until creamy.
Add the granulated sugar and vanilla extract to the cream cheese, beating until smooth.
Add the eggs one at a time, mixing well after each addition. Finally, add the sour cream and mix until combined.
Pour the cheesecake filling over the cooled Rice Krispie crust and smooth the top.
Bake in the preheated oven for 10 minutes. Turn off the oven and leave the cheesecake inside for an additional 30 minutes. This helps prevent cracking.
Remove the cheesecake from the oven and allow it to cool to room temperature. Refrigerate for at least 4 hours or overnight to set.
Before serving, whip the heavy cream with the powdered sugar until stiff peaks form. Spread or pipe the whipped cream over the chilled cheesecake.
Garnish the top with the additional 1 cup of Rice Krispies cereal for added crunch.
Slice and serve chilled.