Unwrap Reese's and place them in the freezer while you prepare the dough.
Heat oven to 350 degrees and prepare a cookie sheet with parchment paper or spray with cooking spray.
In a large bowl (or bowl of a stand mixer) cream together the butter, peanut butter, and brown sugar until light and fluffy, about 3-4 minutes. Mix in the egg and vanilla, scraping down the sides of the bowl as needed.
In a separate bowl, add the flour, baking soda, cornstarch, baking powder, and salt and whisk to combine. Add the flour mixture to the wet ingredients and mix until just combined.
Add the oats, chocolate chips, and M&M candies. Mix on low speed just until dough is combined.
Using a 1/2 cup measuring cup, scoop a dough ball and place it in your hand. Flatten slightly and press a Reese's peanut butter cup in the middle. Fold up the sides of the cookie dough ball and roll into a ball. Place 4 cookie dough balls onto a cookie sheet.
Shape each cookie dough ball into a "cookie shape". Press down the top slightly and shape the sides. The cookies won't spread much while baking so you want to shape then into a cookie shape so they won't bake into rounded balls.
Bake the cookies for 12-14 minutes or until the edges are just starting to brown and the top still looks slightly undercooked. Remove from the oven and cool for 5 minutes on the baking sheet before moving them to a wire rack to cool completely.