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

In a separate bowl, whisk the oats, flour, cinnamon, baking soda, and salt together. Pour into the wet ingredients and beat on low speed until combined, then beat in the add-ins. Dough will be thick.

Cover the cookie dough tightly and refrigerate the dough for at least 30 minutes.

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

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 cookie scoop since the dough can be sticky. I like to press a few extra chopped M&Ms into the tops of the cookies; this is optional, and only for looks.

Bake for 10–11 minutes until very lightly browned on the sides. The centers will look soft and under-baked. Remove from the oven and let cookies cool on the 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.