Melt the butter in a large bowl. Add the brown sugar, white sugar, and oil. Beat with an electric hand mixer at medium speed until smooth. Add the vanilla, egg, yolks, and peanut butter. Beat again.

Add the flour, baking soda, baking powder, and salt. Stir until flour mixture is incorporated. Stir in the M&M's. (I like to reserve half for the top of the cookies.)

Cover the dough and chill for 24-48 hours.

Preheat your oven to 350 degrees. Line cookie sheets with silicone or parchment paper, or spray with cooking spray.

Portion dough into 1/4 cup-sized (2 ounces) dough balls.

Place dough balls on prepared baking sheets about 3 inches apart.

Bake in a preheated oven at 350° for 13-14 minutes or until the middle is set and the edges are barely golden brown. Immediately press a few M&M's on top of each cookie.

Let cookies sit on the pan for at least 5 minutes, then carefully transfer to a wire rack to cool completely. (Cookies are crumbly when warm.)