Prepare baking sheets with parchment paper and set them aside.

Using a stand mixer with a paddle attachment or a handheld mixer, beat the butter on medium-high until it’s smooth, about one minute.

Add the peanut butter and continue beating for about two minutes until the mixture is smooth.

Incorporate the brown sugar and cream the mixture on medium-high for around two minutes until smooth.

Mix in the eggs, one at a time, ensuring each egg is fully incorporated before adding the next. Scrape down the sides of the bowl as needed.

Turn off the mixer and add the flour, oats, baking soda, and salt. Mix on low speed until just combined, being cautious not to overmix.

On low speed or by hand, gently fold in the M&M’s and chocolate chips until just incorporated. If using an electric mixer, mix for only a few seconds.

Spoon out 2-3 tablespoons of dough onto the lined baking sheets, spacing them about 2 inches apart. For uniform cookies, measure the dough. Optionally, top with mini M&M’s.

Chill the dough on the baking sheets in the refrigerator for about 30 minutes. If chilling overnight, cover the dough.

When ready to bake, preheat the oven to 350°F. Remove the cookies from the refrigerator.

Bake for 10 minutes or until the edges are lightly browned and the centers remain soft.

Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.