Coat two 9" x 13" baking pans with cooking spray. Set aside. 

Melt butter in large sauce pan over low heat. Add marshmallows and stir until melted and smooth. Remove from heat.

Add Rice Krispies cereal and stir until evenly coated. I find it best to use a rubber spatula lightly greased with cooking spray for this step. 

Divide the mixture between the two prepared pans and gently press into an even layer. Set aside.

In a double boiler over low heat, melt chocolate chips, stirring until smooth. Evenly spread the melted chocolate on top of one Rice Krispies layer. Remove the second Rice Krispies layer and place on top of the chocolate. Let it sit at room temperature for 1 – 2 hours to allow the chocolate to harden slightly. You can speed up the process by placing in the refrigerator. 

Cut into squares and enjoy!