Preheat the oven to 350F. Line a large baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and brown sugar until light and fluffy.
Add the egg and peanut butter and mix to combine.
Next add the oats and flour and mix to combine again.
Add the M&Ms and fold them into the cookie dough with a rubber spatula.
Scoop 1” balls onto the prepared baking sheet, making sure to place them at least 2” apart.
Bake the cookies for 15 minutes, or just until the edges are set. Then, remove them from the pan and allow them to cool to room temperature on the pan. If using candy eyes and candy corn, press them on the cookies while they’re still warm.
Enjoy immediately. Store any leftover cookies in an airtight container at room temperature for up to five days.
Use quick oats instead of old fashioned oats for a smoother texture.
Replace the light brown sugar with dark brown for more molasses flavor.
Use chunky peanut butter for more crunch inside the cookie.
Make the cookies monster sized by using 2-3 tablespoons of dough in each cookie and adding 1 minute to the bake time.
How to Store
Room Temperature - Place in an airtight container and store for up to 5 days, but I'm sure they'll be eaten before then.
Freezer - If you have any to save, the freezer is the best place for them. Once they've cooled, place them in an airtight, freezer safe container and store for up to 4 months.