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 an extra-large mixing bowl. Carefully sort through and remove any unpopped kernels or hard pieces.
03 - Place butter in a medium saucepan over medium heat. Once melted, add mini marshmallows and stir continuously until completely melted and smooth, about 3-5 minutes.
04 - Pour the melted marshmallow mixture over the popcorn. Working quickly with buttered hands or a spatula, toss until all popcorn is evenly coated with the marshmallow mixture.
05 - Gently fold in candy-coated chocolates, broken pretzel pieces, and roasted peanuts if using. Reserve a small handful of candies for garnishing the top if desired.
06 - Press the mixture firmly and evenly into the prepared pan, using buttered hands or the back of a spatula to compact the popcorn tightly and create a smooth top surface.
07 - Allow the cake to cool completely at room temperature for approximately 1 hour, or until the marshmallow has set and the cake holds its shape.
08 - Carefully invert the pan onto a serving plate. Slice into wedges and serve immediately, or store in an airtight container.