01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick cooking spray, ensuring all surfaces are thoroughly coated to prevent sticking.
02 - Transfer popped popcorn to an extra-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 dissolved and the mixture is smooth and glossy.
04 - Pour the hot marshmallow mixture over the popcorn. Working quickly with buttered hands or a greased spatula, toss and fold until every piece is evenly coated.
05 - Gently fold in candy-coated chocolates, broken pretzel pieces, and peanuts if using. Reserve a small handful of candies for garnishing the finished cake if desired.
06 - Transfer the mixture to the prepared pan. Press firmly and evenly into the pan using buttered hands or a greased spatula, compressing tightly to help the cake hold its shape.
07 - Allow the cake to cool completely at room temperature for approximately 1 hour, or until the mixture is firm and set.
08 - Carefully invert the pan onto a serving plate. Slice into wedges and serve immediately.