Pop the popcorn using your preferred method (air popper, stovetop, or microwave) and place it in a large mixing bowl. Make sure to remove any unpopped kernels.

In a saucepan over medium heat, melt the butter. Once melted, add the granulated sugar and corn syrup. Stir continuously until the mixture comes to a gentle boil. Allow it to boil for about 2 minutes without stirring.

Remove the saucepan from heat and stir in the mini marshmallows until fully melted and combined.
Pour the marshmallow mixture over the popped popcorn and gently stir with a wooden spoon or spatula until all the popcorn is evenly coated.

Allow the mixture to cool slightly (but not too long!). With lightly greased hands or plastic gloves, scoop out a portion of the popcorn mixture and shape it into a ball, about the size of a golf ball. Repeat until all the mixture is shaped into balls.

While the popcorn balls are still slightly warm, roll them in sprinkles or decorative candy to add festive flair. Place the finished popcorn balls on a cookie sheet or parchment paper to cool completely.