Grease a 7x11 or 9x13 pan. Line with parchment paper and grease that as well. I used a 7x11 pan because I wanted a thicker rice crispy treat. You can easily use a 9x13 and make the crispy treat thinner.
In a large saucepan over medium heat, add in the marshmallows, butter, and chocolate syrup. Stir constantly until the marshmallows and butter have melted into a smooth, gooey consistency.
Add in the rice crispies, M&M's, and harvest candy. Stir until everything is coated.
Grease a spatula or your hand depending on what you want to press down with. Press into the prepared pan. Let cool for an hour. Cut into squares and serve.