Line a 9x13-inch dish with parchment paper and set aside.
In a large microwave safe bowl, combine the chocolate and coconut oil. Microwave on high for 2 minutes, stirring every 30 seconds or until melted. If additional time is needed, microwave in 15-20 second increments, stirring between. To the chocolate, stir in 1 cup of marshmallow and mix until melted and incorporated into the chocolate.
Stir the remaining 2 cups of marshmallows and the cereal into the bowl, mixing to coat in the chocolate completely. Transfer mixture to the 9x13 and flatten over the entire bottom of the dish, using the back of a spatula coated with cooking spray. Refrigerate for 2-4 hours or until chocolate has set. Cut into bars; store in the refrigerator.