Line a rectangle baking dish (8x11 - 9x13) with parchment paper or foil. *If you use foil, lightly spray with cooking spray.
Pour marshmallows and butter into a large microwavable bowl. Heat in 1 minute intervals for about 2 1/2 minutes, or until marshmallows have puffed up to about triple their size.
Spray a silicone spatula with cooking spray to help prevent sticking and stir marshmallows with the butter until completely melted.
Stir in rice cereal and mini chocolate chips until cereal marshmallow is evenly distributed.
Press firmly into the prepared dish. Make sure to press together really well (cereal will break).
Allow to cool for one hour before cutting, or refrigerate until firm.