How to Make It
Spray a 9x13-inch baking dish with nonstick cooking spray.
Melt the butter in a large saucepan over medium-low heat. Add the marshmallows and stir until melted. Remove from heat.
Add the Cocoa Puffs and stir with a rubber spatula until combined. Press the mixture into the prepared pan, using a piece of greased parchment paper to flatten the mixture and press it into the corners. Place pan in the freezer for 5 minutes. Meanwhile, remove the ice cream from the freezer to allow to soften.
Remove the pan from the freezer and slice the treats in half across the short side of the pan. Spread Nutella onto one half. Top the Nutella with the ice cream, using a rubber spatula to spread it into an even layer. Lay the other half of the treats on top of the ice cream to form a large sandwich. Return pan to freezer and freeze for at least two hours before cutting into 9 bars.