01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick cooking spray, ensuring all surfaces are coated to prevent sticking.
02 - Transfer popped popcorn to a very large mixing bowl, carefully removing and discarding any unpopped kernels.
03 - In a saucepan over medium heat, melt butter. Add mini marshmallows and stir constantly until completely melted and smooth, approximately 3-5 minutes.
04 - Pour melted marshmallow mixture over popcorn. Using buttered hands or a spatula, quickly toss until popcorn is evenly coated.
05 - Gently fold in candy-coated chocolates, pretzel pieces, and peanuts if using, reserving a handful of candies for garnish if desired.
06 - Firmly press mixture into prepared pan, smoothing top to create even surface.
07 - Allow cake to cool at room temperature for 1 hour or until completely set.
08 - Invert pan onto serving plate, slice into wedges, and serve immediately.