Lay out a large sheet of parchment paper and lightly grease it with cooking spray.

In a large saucepan, melt butter over low heat. Add mini marshmallows and stir until completely melted and smooth. Remove from heat immediately.

Fold in the Rice Krispies cereal until fully coated by the marshmallow mixture.

Transfer the mixture to the greased parchment paper. With greased hands, press it flat into a 1/2-inch thick rectangle.

Melt half of the chocolate chips in the microwave. Spread an even layer of marshmallow fluff over the cereal rectangle, leaving a 1-inch border. Drizzle or spread the melted chocolate over the fluff.

Starting from a long edge, tightly roll the cereal mixture into a log, using the parchment paper to assist you.

Wrap the log tightly in parchment paper and chill in the refrigerator for at least 1 hour until firm.

Unwrap and slice into 1-inch thick rounds using a serrated knife. Melt remaining chocolate chips and drizzle over the top before serving.