In the bowl of a stand mixer, beat coconut oil and sugars together until combined. Beat in egg, egg yolk and vanilla. Set aside. In a medium bowl, whisk together the flour, baking soda and salt. Add the dry ingredients into the wet ingredients and mix together with a rubber spatula or large spoon. Stir in the oats and 1/2 cup of M&Ms. Refrigerate dough for 30 minutes.
Preheat oven to 350 F. Line two large baking sheets with parchment paper or silicone baking mats.
Using a medium cookie scoop (about 2 tablespoons) or your hands, form dough mounds (I made 12) and place on prepared baking sheets, about 2 inches apart. If desired, press a few M&Ms onto the top of each cookie dough ball.
Bake for 9-10 minutes, or until cookies are set but still appearing soft in the centers. Cookies firm up as they cool. Remove from the oven and let cool on baking sheet for about 5 minutes before removing and transferring to a wire rack to cool completely. Store cookies covered at room temperature.