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 sort through and remove any unpopped kernels or hard pieces.
03 - Place butter in a saucepan over medium heat. Once melted, add mini marshmallows and stir continuously until completely melted and smooth, approximately 3-4 minutes.
04 - Pour the melted marshmallow mixture over the popcorn. Work quickly using buttered hands or a spatula to fold and toss until all popcorn is evenly coated with the marshmallow mixture.
05 - Gently fold in candy-coated chocolates, pretzel pieces, and peanuts if using. Reserve a small handful of candies for garnish on the finished cake.
06 - Press the mixture firmly and evenly into the prepared Bundt pan, pressing down firmly to compact. Smooth the top surface with a spatula or buttered hands.
07 - Let the cake cool at room temperature for 1 hour or until completely set and firm to the touch.
08 - Invert the pan onto a serving plate to release the cake. Slice into wedges and serve immediately or store in an airtight container.