Spray a 9x9-inch pan with non-stick cooking spray or grease with butter or line with wax paper.
In a large bowl, microwave the marshmallows and butter for 60 seconds until melted. Add the vanilla and Nutella and stir until just incorporated. Add the rice crispy cereal and fold with a spatula until to mixture just comes together.
Press into the prepared pan with a greased spatula and smooth the top to level. Refrigerate for at least 30 minutes before slicing into squares. Drizzle with melted Nutella, if desired. Enjoy!
Notes
Add ⅛ teaspoon of salt to the recipe if using unsalted butter.
Store in an air-tight container at room temperature or in the refrigerator for up to 5 days.