Preheat the oven to 350ºF and line 2 cookie sheets with parchment paper.
Add the butter to an electric stand mixer, and break it up using the paddle attachment. Add the coconut sugar and peanut butter and beat until combined, scraping down the sides as necessary.
Add the eggs one at a time continuing to beat to thoroughly combine. Scrape down the sides, then add the salt and vanilla and beat again.
Mix together the oats and baking soda, then add them to the cookie dough. Mix just to combine, then add the M&M's and chocolate chips. Stir in the mix-ins, or beat on low to combine.
Portion the cookie dough out in 1.5-3 tablespoons of dough onto a prepared baking sheet. I use a #40 scoop that's 1.5 tablespoon and takes 6-8 minutes to bake, making 5 dozen. Larger scoops take 8-15 minutes.
Lightly flatten the cookies with your hand and decorate the tops with more M&M's if desired. Bake until lightly puffed in the center, crispy on the edges, but still chewy and gooey in the centers.
Let the cookies cool for 5 minutes on the baking sheet, then transfer them to a cooling rack to cool completely before storing.
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Notes
I used natural peanut butter, creamy not chunky, with no added sugar or salt. You can also use cashew butter or almond butter as well.I use the Unreal brand of M&M's which are made with all-natural coloring. You can also replace the M&M's with additional chocolate chips.To store - these cookies are best right away but will keep for 2-3 days in a tightly sealed container at room temperature. They will get drier as they sit out.Freeze - I freeze all my leftover cookies right away to keep them fresher. Place them into an airtight sealed freezer-safe container and freeze them for up to 1 month. Simply thaw at room temperature and enjoy!For giant monster cookies, use 3 tablespoons of dough per cookie and increase the baking time to 10-15 minutes. This recipe makes 5 dozen 1.5 tablespoon cookies or around 2 dozen larger cookies.