In a medium bowl, whisk together flour, oats, baking soda and salt. Set aside.
In the bowl of a stand mixer (or using a hand mixer), beat butter, brown sugar and sugar on medium-high speed until light in color, about 3 minutes. Add peanut butter, egg, egg yolk and vanilla. Mix until incorporated.
Add flour mixture to wet ingredients and mix just until combined. Stir in chocolate chips and M&M’s by hand.
Cover the dough and chill for 2 hours and up to overnight.
Preheat oven to 350° F. Line a baking sheet with parchment paper and set aside.
Use a medium cookie scoop to scoop dough and roll into balls. Place on baking sheet 1 ½ inches apart. Bake at 350° F for 10 to 12 minutes or until the edges turn golden. Cool on the pan for a few minutes, then transfer to a wire rack to cool completely.
Notes
No-stir peanut butter is a must. Do not use the kind where the peanut butter and oil separate as this will significantly affect the texture of the cookie.Store cookies in an airtight container at room temperature for up to 1 week.Freeze dough for up to 3 months. Wrap dough in plastic wrap and store in the freezer. Defrost in the refrigerator before scooping and baking.