Microwave popcorn according to package directions. Line a rimmed baking pan with nonstick foil. Spread popcorn onto foil-lined pan.
Melt butter in a small saucepan over medium heat. Add sugar and corn syrup. Increase heat to high and bring to a boil, stirring constantly. Boil until sugar dissolves, about 2 minutes. Cool 4 minutes.
Pour sugar mixture over popcorn on baking pan. Coat a spatula with nonstick cooking spray. Push hot mixture into center of baking pan, folding over to combine. Coat your hands with nonstick cooking spray and sprinkle popcorn mixture with candies or seeds. Using your hands, quickly combine ingredients and shape into firmly packed 2-inch balls. Cool completely. Wrap each ball in plastic wrap, or keep covered until ready to serve.