01 - Generously grease a 10-inch Bundt pan or tube pan with butter or nonstick spray for easy release.
02 - Transfer popped popcorn into a large mixing bowl, removing any unpopped kernels to avoid hard bites.
03 - In a saucepan over medium heat, melt the unsalted butter, then add mini marshmallows, stirring continuously until fully smooth and combined.
04 - Drizzle the melted marshmallow mixture evenly over the popcorn. Using buttered hands or a spatula, swiftly mix until all popcorn is thoroughly coated.
05 - Gently fold in the candy-coated chocolate pieces, pretzel pieces, and peanuts if using, reserving a small portion of candies for garnish if desired.
06 - Firmly press the mixture into the prepared pan, ensuring an even, compact layer and a smooth top.
07 - Allow to cool at room temperature for 1 hour, or until completely set.
08 - Invert the cake onto a serving plate, slice into portions, and serve immediately.