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

Add the butter and mini marshmallows to a microwave-safe bowl. Heat in 30-second intervals, stirring until melted and smooth.

In a large bowl, add the popped popcorn.

Pour the melted marshmallow mixture over the popcorn and stir with a wooden spoon until coated.

Sprinkle in the M&M’s gently to avoid smudging the colors.

Scoop the coated popcorn into the prepared Bundt pan, pressing down gently.

Allow to rest at room temperature for 2 hours until firm.

Turn onto a serving plate and cut into 12 slices.