Spray 11x7 baking pan with nonstick spray. Set aside.
In a large saucepan, melt the butter. Pour in the marshmallows and constantly stir until melted with butter.
Remove the pan from heat and pour in the rice krispie cereal and graham cracker pieces. Mix together well.
Add chocolate chips and gently stir - you want most of the chips to hold their shape and not to completely melt. Immediately press rice krispie treats into prepared baking pan. Use the flat side of a spatula sprayed with nonstick spray to flatten the rice krispie treats down evenly.
Refrigerate for about 25 minutes and cut into 24 squares.