Beat together the cake mix, eggs, and vegetable oil until combined. Add the food coloring and beat until well mixed.
Stir in the M&Ms and chocolate chips.
Cover and chill for at least 30 minutes.
When ready to bake, preheat the oven to 350 degrees and line a baking sheet with parchment paper.
Use a tablespoon to scoop a heaping spoonful of dough. Roll the dough into a ball and place it on the lined baking sheet. Press the dough down slightly, then decorate with additional candy and chocolate chips if desired. Continue until all the dough has been used.
Bake the cookies in the preheated oven for 10 to 15 minutes, until the edges of the cookies are slightly brown.
Allow the cookies to cool on the baking sheet for five minutes, then transfer to a wire cooling rack to cool completely before serving.
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Notes
Do not rush the chilling of this cookie dough. It's very important that you make sure that the cookie dough chills for at least 30 minutes.
If you don't have parchment paper on hand, you can use a silicone baking mat instead.
If you want even-sized cookies you can weigh them with a kitchen scale so that all your cookies weigh the same, or use a cookie scoop.
To get perfectly shaped cookies, use a spatula or a glass to gently round the edges of the cookies while they are still warm.