01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick cooking spray to prevent sticking.
02 - Place popped popcorn in an extra-large mixing bowl. Carefully 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, about 3-5 minutes.
04 - Pour the melted marshmallow mixture over the popcorn. Using buttered hands or a spatula, quickly toss until all popcorn is evenly coated.
05 - Gently fold in candy-coated chocolates, pretzel pieces, and peanuts if using. Reserve a small handful of candies for garnish if desired.
06 - Firmly press the mixture into the prepared Bundt pan, packing it tightly and smoothing the top surface.
07 - Let the cake cool at room temperature for 1 hour or until completely set and firm.
08 - Invert the pan onto a serving plate, lift away the pan, slice into wedges, and serve immediately.