Melt butter in a very large pot over medium-low heat until the butter melts completely
Add the marshmallows and stir constantly over medium heat until fully melted
Quickly fold in the crispy rice cereal until coated in the cereal mixture
Transfer to a prepared pan (sprayed with cooking spray or lined with parchment paper)
Press mixture gently with a buttered spatula into an even layer at the bottom of the pan
Let cool at room temperature before slicing into squares
If you add melted chocolate to the top, place in fridge for 1 hour until chocolate has cooled
Keep in airtight container for up to 3 days