In a large bowl, use an electric mixer to combine the cake mix, oil, eggs and vanilla until well-combined. Stir in 1 cup of the M&M's. Cover the bowl and chill for at least 30 minutes.
Preheat the oven to 350℉. Line baking sheets with parchment paper or silpat mats.
Roll the dough into balls and place the balls two inches apart on the prepared baking sheets. Flatten the balls slightly with the palm of your hand, and dot the tops of the cookies with additional M&M's.
Bake for 9 to 11 minutes, just until set.
Notes
You can use regular sized M&M's or miniature M&M's.