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

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

In a large bowl, add the popped popcorn.

Pour the melted marshmallow mixture over the popcorn and stir until coated.

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

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

Allow the cake to sit at room temperature for 2 hours to set.

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