Line a baking sheet with wax paper, spray the paper with nonstick cooking spray, and set the sheet aside.

Put 20 cups popped popcorn into a very large mixing bowl and set aside.

In a medium saucepan combine 3/4 cup light corn syrup, 1/4 cup butter, 2 teaspoons cold water, the powdered sugar called for in the ingredient list, and the marshmallows (use 1 cup mini marshmallows or 14 large marshmallows).

Heat the mixture over medium-high heat, stirring almost constantly, until it comes to a boil and all marshmallows are completely melted and the mixture is smooth. Do not leave unattended to avoid burning.

Remove the saucepan from the heat. Immediately drizzle the hot marshmallow mixture evenly over the popcorn.

Using a heatproof spatula or wooden spoon, fold and toss the popcorn gently but thoroughly until the popcorn is evenly coated with the marshmallow mixture.

Butter your hands with the 2 tablespoons of softened butter reserved for shaping (butter your whole palms and fingers). Working carefully—the mixture will be hot—take portions of the coated popcorn and shape into balls about the size of a tennis ball, squeezing gently to compact them.

Place each formed popcorn ball on the prepared baking sheet. Reapply softened butter to your hands as needed and continue shaping until you have about 16–20 popcorn balls.

Let the popcorn balls cool and set at room temperature until they are no longer hot and hold their shape. Wrap individually in plastic wrap to store at room temperature, or serve immediately.