01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick cooking spray, ensuring all surfaces are well coated to prevent sticking.
02 - Transfer the popped popcorn to a very large mixing bowl. Carefully sort through and discard any unpopped kernels or hard pieces.
03 - Place the butter in a saucepan over medium heat. Once melted, add the mini marshmallows. Stir continuously until completely melted and smooth, approximately 3-5 minutes.
04 - Pour the melted marshmallow mixture evenly over the popcorn. Work quickly using buttered hands or a spatula to toss until all popcorn is thoroughly coated.
05 - Gently fold in the candy-coated chocolates, broken 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, pressing down firmly to eliminate air pockets. Smooth the top surface evenly.
07 - Allow the cake to cool at room temperature for 1 hour 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.