01 -
Lightly spray a 10-cup Bundt cake pan with nonstick cooking spray.
02 -
Place the salted butter and mini marshmallows in a medium microwave-safe bowl. Heat on high in 30-second intervals, stirring between each, until the mixture is completely melted and smooth.
03 -
Transfer the pre-popped popcorn to an extra-large bowl (6 to 7 quarts).
04 -
Pour the melted marshmallow and butter mixture over the popcorn. Gently stir using a wooden spoon to evenly coat the popcorn.
05 -
Sprinkle the plain M&M's over the coated popcorn and gently fold them in to avoid smudging the colors.
06 -
Scoop the mixture into the prepared Bundt pan, gently pressing down as you fill to compact the mixture evenly.
07 -
Allow the mixture to rest at room temperature for 2 hours until firm. Turn the cake onto a serving plate and slice into 12 portions.