Line a 8x8-inch baking dish with wax paper or parchment. Set aside.
In a large skillet or Dutch oven (I used a non-stick skillet for easy cleaning), set over medium-low heat, add the butter. When melted, add the marshmallows and stir until smooth, about 5 minutes.
Turn off the heat and mix in the rice krispy cereal and crushed cinnamon toast crunch. Mix until the cereal is completely coated in the marshmallow. Quickly transfer and press into the prepared baking dish. I usually use a silicon spatula and press it down firmly so it reaches each of the edges and corners. Allow to cool and then slice and serve.
*You can either add the cinnamon toast cereal to a bowl and crunch it with your hands. OR you can add it to a ziploc plastic bag and smash it in the bag.