Preheat the oven to 300°F. and line a cookie sheet with parchment paper. *See Notes
Using an electric hand mixer or stand mixer fitted with a beater attachment, cream together butter, white sugar and brown sugar in a mixing bowl.
Add the peanut butter, egg, and vanilla, and mix well on high speed to combine.
Reduce speed to low and add in flour, baking soda, and salt. Mix just to combine but don’t overmix or your cookies will be tough and dry.
Using a wooden spoon or silicone mixing spatula, fold in rolled oats, 1 cup M&Ms, and mini chocolate chips.
Using a 3-tablespoon cookie scoop, portion out the dough and arrange the balls about 2 inches apart on the prepared cookie sheet.
Bake for 20-22 minutes, or until the edges start to brown and the center is soft.
Remove cookies from the oven and while still warm, sprinkle with extra M&Ms (optional).
Let the cookies cool on the cookie sheet for 20-30 minutes. This allows the center and chocolate to set. If you prefer, serve while warm and gooey.
Notes
A 3-tablespoon cookie scoop makes cookies that are a little bit larger than traditional cookies. For this reason, they need to bake at a lower temperature for a longer period of time. Otherwise, they’ll burn on the outside before the inside is cooked. If you prefer to use a traditional 2-tablespoon sized cookie scoop, increase oven heat to 350°F. and bake the cookies for 12-15 minutes.