In a large mixing bowl, add cereal. Set aside.
Grease an 8" x 8" baking dish and set aside.
In a small pot over medium heat, melt butter and chopped Mars Bars. Stir until completely melted and smooth, then remove from heat.
Immediately mix melted Mars Bars mixture with cereal, stirring until all the cereal is coated.
Pour into prepared baking dish, and press mixture down with a spatula or large spoon to make an even layer. Refrigerate while you make the chocolate layer.
In a small bowl, add chocolate chips and butter. Use a microwave to melt, 30 seconds at a time, stirring in between until melted and smooth.
Pour melted chocolate mixture over Mars Bars squares. Cover and refrigerate until ready to serve, then cut into 32 squares.