Place your butter into your stand mixer with the paddle attachment secured. Let your mixer run for about a minute to soften your butter. Then, add in your peanut butter and continue mixing until your mixture is fully combined, smooth and lightened in color. Scrape down the sides of your bowl and paddle.
Add in both sugars, and turn your mixer back on for a minute or two. Scrape down the sides of your bowl and paddle again. Then, add in your egg and vanilla extract and mix for 2 to 3 minutes until your mixture is lightened in color, scraping down your bowl and paddle about halfway through, and being sure to reach down to the bottom of your bowl.
In a separate bowl, measure and mix your dry ingredients, flour, oats, baking soda, and salt. Add these dry ingredients into your stand mixer, and turn your mixer on at a low speed to start. Once your dough is about halfway formed, pause your mixer and scrape down to the bottom of the bowl with your spatula.
Measure out your semisweet chocolate chips, peanut butter chips, and M&Ms. I like to chop or partially crush my M&Ms but that is baker's choice! Add all of your mix ins to your dough, and turn your mixer back on at a low to medium speed for about 30 seconds or until your dough has just formed and your mix ins are evenly dispersed. Note, you can save some of your mix ins to top your cookie scoops with later.
your dough in your bowl by gently pressing plastic wrap onto the top layer of your dough, or transfer to an airtight container or into plastic wrap. Place your dough into the fridge and chill for 6 hours or overnight. Note: you can do a shorter chill (e.g. 1 hour) but just know your cookies may spread more than we want.
When you're ready to bake, preheat your oven to 350 degrees. Onto a lined cookie sheet or two (I used parchment paper for my liner) use a cookie scoop or large spoon to portion out your dough. It should make about 14 scoops, 2 1/2 - 3 tablespoons per cookie. Use your palm to gently flatten each cookie scoop, and if you saved some mix ins you can press them into the top of our dough scoops now.
Bake your cookies for 9 to 12 minutes, allow them to cool slightly, and enjoy!