Preheat oven to 350°F. In a stand mixer bowl with the paddle attachment on medium speed, cream together butter and both sugars for about 2 minutes. Add peanut butter and mix until well combined while scraping the sides of the bowl with a rubber spatula frequently.

Add egg yolk and vanilla extract to the butter mixture and mix on low until just combined. In a separate small mixing bowl, whisk together flour, salt, and baking soda. Add flour mixture to butter mixture all at once and mix until just combined. Fold in M&M's until well distributed.

Scoop dough onto parchment paper lined cookie sheet, they won't REALLY spread so make them the size that you would like still leaving about 1 inch away from each other. Also, when scooping, don't form with your hands because then they will spread due to the warmth of your hands. Bake for 12-14 minutes (11-12 minutes for more gooey cookies, 13-14 minutes for a chewy but stiffer cookies) Let cool on cookie sheet for at least 3 minutes before transferring to cooling rack- remember the longer they are on the hot cookie sheet, the more they will continue to cook.