Add the cake mix, vegetable oil, and eggs to a large bowl or stand mixer fitted with a paddle attachment and beat until combined.
Fold in half of the M&M’s.
Chill the dough for 60 minutes. Do not skip this step!
Preheat the oven to 350 degrees F and line a baking sheet with parchment paper or a silicone baking mat.
Use a medium cookie scoop to portion out the dough and gently roll into balls. Then roll the balls in the sugar to coat. Place the balls of dough on the prepared baking sheet at least 2 inches apart.
Bake for 8 to 9 minutes. Do not over bake these cookies, they will look puffy and slightly underdone, but they will finish cooking on the baking sheet after coming out of the oven.
Press the M&M’s into the tops of the cookies right after they come out of the oven, then let the cookies cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.
It is extremely important that you chill these cookies for at least 45 minutes and not longer than 1 hour. The dough will be too soft to shape and roll without chilling. Prior to the 1-hour mark these cookies really puff up and have more of a soft and chewy texture, after 1 hour they can become hard and crunchy.
Store in an airtight container for up to 1 week or freeze for up to 3 months.