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

Add the butter and mini marshmallows to a microwave-safe bowl and 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 well combined.

Gently fold in the M&Ms, being careful not to smudge the colors.

Scoop the mixture into the prepared Bundt pan and press down gently.

Let the cake rest at room temperature for about 2 hours until firm.

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