01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick cooking spray, ensuring all surfaces are coated for easy release.
02 - Transfer popped popcorn to an extra-large mixing bowl. Carefully sort through and discard any unpopped kernels or hard pieces.
03 - In a medium saucepan over medium heat, melt butter completely. Add marshmallows and stir constantly until fully melted and smooth, approximately 3-4 minutes.
04 - Pour melted marshmallow mixture over popcorn. Working quickly with buttered hands or a spatula, toss until every piece 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 - Press mixture firmly and evenly into prepared pan, using buttered hands or spatula to compact the mixture and create a smooth surface.
07 - Allow cake to cool completely at room temperature for 1 hour until fully set and firm.
08 - Invert pan onto serving plate to release cake. Slice into wedges and serve immediately.