From the cookbook Mug Meals by Dina Cheney.
Place the butter in a 12-oz. mug, cover, and microwave until melted, about 40 seconds.
Spray the inside of the mug with cooking spray. Stir in the cereal, marshmallows, and fudge sauce, mixing well. Cover and microwave until the marshmallows have fully melted, about 1 minute (check after 40 seconds).
Divide into two piles on a piece of waxed paper or parchment. Form into 2-inch-diameter "cookie" shapes and refrigerate until cool and solid, about 30 minutes.