Preheat the oven to 350°F. Line baking sheets with parchment paper or silicone baking mats and set aside.
In a large mixing bowl, combine the cake mix, eggs, melted butter, and vanilla extract. Stir until well combined and a thick dough forms.
15.25 ounce box Devil's Food Cake Mix, 2 large eggs, 1/2 cup butter, 1 teaspoon vanilla extract
Gently fold in the mini chocolate chips and M&M's candies until evenly distributed throughout the dough.
1/2 cup mini chocolate chips, 1/2 cup mini M&M candies
Cover the mixing bowl with plastic wrap and refrigerate the dough for at least 30 minutes, or overnight. Chilling the dough will enhance flavor and texture.
Once chilled, using a medium- size cookie scoop, scoop out portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Bake for 10-12 minutes, or until the edges are set and the tops of the cookies have a slight crust. Do not overbake. The cookies may look doughy in the center but will firm up as they cool. This is what makes them stay soft.
Allow the cookies to cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely