Using a hand mixer or a stand mixer fitted with a paddle attachment, cream the sugars with the butter on medium speed (it will take around 2 minutes). Then at high speed, mix in the egg, the peanut butter, and the vanilla extract (about 2 minutes). Set aside.
Add the flour and the baking soda, and mix on low speed until just combined. Fold in the oats, the M&M's, and chocolate chips.
Preheat oven to 430°F (220°C) and line two large baking sheets with parchment paper or silicone baking mats. Set aside.
Using a medium cookie scoop, scoop out the batter and place on baking sheets. Keep about 2 inches between the cookies as they will spread.
Bake for 8-10 minutes. The cookies will look soft and slightly under-baked. Remove them from the oven and allow them to cool down for 5 minutes.
Transfer to a wire rack to completely cool down.
I recommend using mini M&Ms so you get more of them in each bite, but regular M&Ms work in this recipe too. If you only have larger M&Ms, such as peanut M&Ms or peanut butter M&Ms, I recommend chopping them into smaller pieces.
Use an electric beater to cream the butter with the sugar.
The butter MUST be softened to room temperature, and the egg must be at room temperature as well. Remember that butter at room temperature is imperative in the creaming butter with sugar step.
Feel free to add butterscotch morsels, white chocolate chips, or peanut butter chips to the cookie dough.
Use a cookie scoop to create the cookie balls, it's easier and your cookies will be the same size.
The cookies can be kept in a sealed container at room temperature for up to 5 days, in the fridge for up to 7 days, or in the freezer for up to 2 months.