Line a mini-muffin pan with cupcake liners, or spray with cooking spray.
Combine marshmallows, peanut butter, butter and milk in a medium-sized glass bowl. Microwave on high for 1 minute 30 seconds. Stir until mostly smooth. Add powdered sugar, stir until combined. Fold in cereal.
Scoop about 1 rounded tablespoon into each muffin cup. I used a cookie scoop for this- makes it super fast and easy! Gently press down with a greased spoon to flatten.
Spoon about 2 teaspoons of melted chocolate* onto the top of each peanut butter bite. Top with sprinkles, if you'd like. Let treats cool so that chocolate can harden and enjoy! You can really tell they have Krispies in them the first day, but after that, they get softer and are less apparent.