Using a hand mixer or a stand mixer fitted with paddle attachment, cream the softened butter and both sugars together on medium speed until smooth, about 2 minutes. Add the eggs and mix on high until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed. Add the vanilla and molasses and mix on high until combined. Set aside.

In a separate bowl, whisk the flour, baking soda, cinnamon, and salt together. Add to the wet ingredients and mix on low until combined. Beat in the oats and M&Ms on low speed. Dough will be thick, yet very sticky. Chill the dough for 30-60 minutes in the refrigerator (do the full hour if you’re afraid of the cookies spreading too much). If chilling for longer (up to 2 days), allow to sit at room temperature for at least 30 minutes before rolling and baking.

Preheat oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats. Set aside.

Roll balls of dough (about 1 and 1/2 Tablespoons of dough per cookie) and place 2 inches apart on the baking sheets. I recommend using a medium cookie scoop since the dough can be sticky. Bake for 11-12 minutes or until lightly browned on the sides. The centers will look soft and under-baked. Remove from the oven and let cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely. The cookies will continue to “set” on the baking sheet during this time.

Cover and store leftover cookies at room temperature for up to 1 week.