In a small bowl, combine flour, baking soda, sea salt, and cinnamon. Set aside.
In the bowl of a standing mixer fitted with the paddle attachment, beat together butter, peanut butter, brown sugar and granulated sugar until light and fluffy.
Add vanilla and egg and beat until combined.
Add the dry ingredient mixture and beat until combined.
Add oatmeal and beat until evenly distributed.
Add chocolate chips and M&Ms and stir until evenly distributed.
Cover dough with plastic wrap, or transfer to an airtight container with a lid and chill for at least 1 hour.
After 1 hour, preheat oven to 350 degrees F. Line a large baking sheet with parchment paper.
Use a small cookie scoop to portion dough into 1 ½ TBS portions, roll into balls and place evenly spaced on the prepared baking sheet. Gently press the tops down slightly. (NOTE: Dough will be slightly tacky, but should still roll easily into balls).
Bake in the preheated oven for 10-11 minutes, or until cookies are just barely set.
Let cool on the baking pan for 5-10 minutes, then transfer to a wire cooling rack to cool completely.
Form dough into balls.
Place cookie dough balls on a non-stick cookie sheet and put them in the freezer for 1-2 hours or until firm.
Transfer to an airtight, freezer-friendly container (I recommend either a glass storage container or a freezer-friendly ziplock bag).
Freeze for up to 1 month.
To bake: remove dough from the freezer and let it come to room temperature. Then bake according the the recipe instructions.
Store these monster cookies in an airtight container at room temperature for up to 3 days, or in the refrigerator for 5 days. You can freeze baked cookies for up to 1 month in an airtight container.
Salted Butter. Unsalted butter or coconut oil can be used in place of salted butter.
Creamy peanut butter. I recommend using a variety of peanut butter that is solid at room temperature. You can also use crunchy peanut butter if you like the little pieces of nuts in your monster cookies.
Eggs. Try to make sure the egg is room temperature! I have not made this recipe with an egg substitute, so if you do let me know in the comments below!
Light brown sugar. If you like a little bolder, molasses taste then you can use dark brown sugar. Coconut sugar also works well.
Granulated Sugar. Organic cane sugar is my granulated sugar of choice, but regular granulated sugar works perfectly.
All-purpose flour. A 1:1 gluten-free all purpose flour can be used to make these cookies gluten-free.
Fine sea salt. I only ever bake with pure, fine sea salt. If you are using iodized salt I recommend decreasing the amount of salt by half.
Chocolate chips/M&Ms. The traditional mix-ins in monster cookies are chocolate chips and M&Ms. However you can choose to use whatever you'd like. Chopped peanut butter cups, peanut butter chips, white chocolate chips, etc.