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 a very large mixing bowl. Carefully sort through and remove any unpopped kernels.
03 - Heat butter in a saucepan over medium heat until melted. Add mini marshmallows and stir continuously until completely melted and smooth, approximately 3-4 minutes.
04 - Pour melted marshmallow mixture over popcorn. Work quickly using buttered hands or a spatula to toss until all popcorn is evenly coated.
05 - Gently fold in candy-coated chocolates, pretzel pieces, and peanuts if using. Reserve a small handful of candies for topping decoration.
06 - Press mixture firmly and evenly into prepared pan, packing tightly to help the cake hold its shape. Smooth the top surface.
07 - Let cake cool at room temperature for 1 hour until completely set and firm to the touch.
08 - Invert pan onto serving plate to release cake. Slice into wedges and serve immediately.