Cream the softened butter and both sugars together
on medium speed until smooth.
Add the egg and vanilla and mix on high until combined, about 2 minutes.
Scrape down the sides and bottom of the bowl as needed. Set aside.
In a separate bowl, toss the oats, flour, cinnamon, baking soda, and salt together.
Add to the wet ingredients
and mix gently until combined.
Chill the dough for at least 30 minutes in the refrigerator.
Preheat oven to 325F degrees. Line two large baking sheets with parchment paper or silicone baking mats. Roll balls of dough (about 1 tablespoon of dough per cookie) and place 2 inches apart on the baking sheets. Yes, my little one decided to stay by the side to make sure he gets to keep his cookies and eat it too!
He pressed a few extra M&Ms into the tops of the cookies. He probably would have stuffed some more if not for his mommy insisting on some damned blog post looking palatable like anyone who cares!
Bake for 10-11 minutes until very lightly browned on the sides. The centers will look very soft and might appear undone but they will firm as they cool.
Remove from the oven and let cool on baking sheet for 5 minutes before transferring to a wire rack.