In a large bowl, melt butter in the microwave. Add mini marshmallows and stir to coat. Microwave for 45 seconds and stir until smooth. (If the marshmallows aren’t completely melted, microwave for an additional 15 seconds.)
Mix in chocolate crispy rice cereal and gently stir until well coated.
Drop by large spoonfuls over the marshmallow cream layer and carefully spread out evenly. (I put my hand in plastic baggie with a little butter on it and gently spread it out.)
Firmly press the top layer down into the marshmallow cream to really hold things together.
Cover the bars and cool completely in the fridge.