01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick spray to ensure easy release.
02 - Place the popped popcorn in a large mixing bowl. Remove any unpopped kernels to prevent biting into hard pieces.
03 - In a saucepan over medium heat, melt butter completely, then add mini marshmallows. Stir continuously until all marshmallows are fully melted and the mixture is smooth.
04 - Pour the hot marshmallow mixture over the popcorn. With a buttered spatula or buttered hands, quickly mix until all popcorn is evenly coated.
05 - Gently fold in candy-coated chocolate pieces, pretzel fragments, and peanuts if using. Reserve a small portion of candies for topping if desired.
06 - Firmly press the mixture into the prepared Bundt or tube pan, ensuring an even, compact layer and a smooth surface.
07 - Allow the cake to cool at room temperature for approximately 1 hour or until fully set. Invert the pan onto a serving plate to unmold.
08 - Slice into portions and serve immediately or store in an airtight container for up to three days.