Pop popcorn and spread out on a paper towel, removing any kernels or burnt pieces, and place in a large bowl. Mix in mini M&Ms.
Melt white chocolate in the microwave in 30 second intervals, stirring in between, until melted and smooth. Stir in cake mix until smooth, then stir in about 3 teaspoons of sprinkles. Immediately pour over popcorn. Using a wooden spoon, mix the popcorn and M&Ms until completely coated.
Pour out onto wax paper or into a wax-paper lined jelly roll pan and spread into a single layer. Sprinkle with sprinkles until festive, then let cool and harden. Break into pieces and enjoy!