In a large skillet over low heat, add butter and marshmallows. Cook, stirring frequently until marshmallows are almost all melted but still slightly lumpy. Then, immediately remove from heat.
Pour rice crispy's into the marshmallow mixture and mix to combine. Then, using a lightly buttered rubber spatula, press rice crispy's into the parchment paper lined baking dish. Let cool.
Make the chocolate crunch bar topping:
In a small microwave safe bowl, add chocolate chips and microwave for about 15 seconds. Remove from microwave and stir frequently. Place back in the microwave for 10 second increments until chocolate is melted but be very careful not to burn. Between each session stir chocolate frequently. Pour remaining rice crispy's into the chocolate and stir to combine.
Evenly spread chocolate crunch topping directly on top of the rice crispy treats. Place in the fridge for about 10 minutes to harden quickly. Then remove from fridge, slice into squares and serve. *Storing instructions below.
Notes
Store rice crispy treats on the counter wrapped tightly in plastic wrap or in a tightly sealed storage container on the counter for up to 3-4 days.