Sift together the flour, cake mix, baking soda, and salt into a medium bowl and set aside.
In a large bowl or stand mixer fitted with a paddle attachment, cream together the softened butter and both sugars until light and fluffy.
Add the egg and beat for 1 minute on high, then add in the vanilla. Scrape down the sides of the bowl as needed.
Add in the dry ingredients and mix just untiil combined.
Fold in the M&M's and sprinkles with a rubber spatula.
Cover the dough and chill for at least 2 hours.
After chilling, preheat the oven to 350 degrees F and use a medium cookie scoop to measure the dough into balls.
Roll the dough between your palms to create tall balls of cookie dough and place them about 3 inches apart on a parchment lined cookie sheet.
Press some additional M&M's into the tops of the balls and bake for 10 to 12 minutes.