Line a 9-inch x 13-inch baking dish with parchment paper, making sure some foil is hanging over the edges of the dish. Spray the foil with non-stick cooking spray. Set aside.
Place a large pot over medium heat. Add the butter. Once melted, add the marshmallows and cocoa powder to the pot. Stir frequently until the marshmallows are completely melted.
Remove the pot from the heat and stir in the chocolate chips until melted and combined. Pour in the rice krispies and stir until evenly coated in the chocolate marshmallow mixture.
Transfer the chocolate rice krispies to the foil-lined dish and press down into an even layer. You can use your hands (be careful, they're hot!) or a rubber spatula.
Allow the treats to cool completely, 30 minutes to 1 hour.
Mint Frosting
Once the treats are set, add the butter to the bowl of a stand mixer (or large mixing bowl with a hand mixer). Run on low speed and add the powdered sugar 1 cup at a time, mixing between additions.
Once the frosting pulls away from the sides of the bowl, turn the mixer to high and beat until it lightens in color and becomes fluffy.
Pour in the water, peppermint extract, and food coloring. Mix until evenly combined and the frosting is green. (You can add more food coloring until your desired shade of green is reached.) Spread the frosting over the top of the rice krispie treats.
Use the foil to lift the treat from the baking dish. Cut into squares - I do a 4x5 grid for 20 treats.
Notes
Store the treats (or leftovers) in the refrigerator until you're ready to serve them. Place the treats on the counter 30 minutes before serving to bring them back up to room temperature.