Preheat your oven to 350°F and line baking sheets with parchment paper.
In a large bowl, mix together the cake mix, eggs, oil, and vanilla extract until well combined. The dough will be thick but soft.
Gently fold in the M&M's or sprinkles, saving a few to press on top of each cookie.
Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart. Press a few extra M&M's or sprinkles on top of each cookie.
Bake for 9-11 minutes, until the edges are just set. The centers will look slightly underdone—that's perfect!
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.