1. Pop popcorn according to package directions. Let cool and then remove any unpopped kernels. Set aside.
2. Spray a 9×13 pan with cooking spray, set aside.
3. In a large pan, melt butter over medium low heat. Once melted, brush butter around pan and up sides of pan. Add marshmallows and continue to heat, stirring, until marshmallows are completely melted.
4. Remove from heat and fold in popcorn.
5. Press coated popcorn into prepared 9×13 pan.
6. Place chocolate in microwave safe dish and heat on high in microwave for 50 seconds. Let cool 1 minute and then stir until no lumps remain. If needed heat for an additional 15 seconds.
7. Drizzle chocolate over popcorn and then immediately sprinkle with sprinkles and M&Ms.
8. Once chocolate is set, cut into squares using a sharp knife sprayed with cooking spray.

Notes

For easy pressing popcorn into pan, cover one hand with a large plastic bag, spray bag well with cooking spray then us bag covered hand to press popcorn into pan.