01 - Line a large baking sheet with parchment paper and lightly grease your hands with butter or nonstick spray.
02 - Place popcorn in a large mixing bowl, ensuring all unpopped kernels are discarded.
03 - In a medium saucepan over medium heat, melt butter. Add corn syrup, granulated sugar, and salt; stir constantly until the mixture reaches a gentle boil. Continue boiling for 2 to 3 minutes, or until sugar is completely dissolved and syrup appears clear.
04 - Remove saucepan from heat and incorporate vanilla extract, stirring thoroughly.
05 - Immediately pour hot syrup over popcorn. Quickly mix with a wooden spoon or heatproof spatula to ensure each kernel is evenly coated.
06 - Gently fold in candy corn and mini marshmallows, if using, until distributed throughout the popcorn.
07 - When the mixture is cool enough to touch yet still moldable, roughly 2 to 3 minutes after stirring in add-ins, shape into 3-inch popcorn balls using greased hands. Compress gently to hold shape without crushing.
08 - Arrange popcorn balls on the prepared baking sheet and allow to cool completely before serving or packaging.