Preheat your oven to 350°F (175°C). This ensures your cookies bake evenly.
In a large mixing bowl, combine the peanut butter, packed brown sugar, and granulated sugar. Mix until well combined and creamy.
Crack the large egg into the mixture and stir until fully incorporated. This will help bind your cookies together.
Gently fold in the chopped Reese's Peanut Butter Cup Miniatures and the M&Ms into the cookie dough. Make sure they are evenly distributed throughout the mixture.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a baking sheet lined with parchment paper, leaving some space between each cookie as they will spread while baking.
Bake the cookies in the preheated oven for 10-12 minutes or until the edges are lightly golden but the centers are still soft.
Remove the baking sheet from the oven and let the cookies cool on the sheet for 5 minutes before transferring them to a cooling rack. This allows them to set and makes them easier to handle.
