01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick cooking spray.
02 - Place popped popcorn in a very large mixing bowl. Sort through and discard any unpopped kernels.
03 - In a saucepan over medium heat, melt the butter. Add mini marshmallows and stir constantly until completely melted and smooth.
04 - Pour the melted marshmallow mixture over the popcorn. Using buttered hands or a spatula, quickly mix until the popcorn is evenly coated.
05 - Gently fold in candy-coated chocolates, pretzel pieces, and peanuts (if using), reserving a handful of candies for decoration if desired.
06 - Press the mixture firmly into the prepared pan, smoothing the top evenly.
07 - Let the cake cool at room temperature for 1 hour or until completely set.
08 - Invert onto a serving plate, slice into wedges, and serve.