Lightly spray a 10 cup Bundt cake pan with nonstick cooking spray.

Add the butter and both packages of the mini marshmallows to a medium size microwave safe bowl. Heat on high power in 30 second increments, stirring in between increments until the butter and mini marshmallows are completely melted and smooth.

In an extra large bowl (6 to 7 quarts) add the popped popcorn.

Pour the melted mini marshmallow mixture over the popcorn and use a wooden spoon to stir the mixture together.

Sprinkle in the M&M’s. Try to be as gentle as possible when stirring as the coloring of the M&M’s can smudge and run.

Scoop the coated popcorn into the prepared Bundt cake pan. Gently press the popcorn mixture down as you scoop more coated popcorn into the pan.

Allow the popcorn cake to rest at room temperature for 2 hours, or until firm.

Turn the popcorn cake onto a serving plate, and cut into 12 slices.