Put popped popcorn in a large bowl and set aside.

In a large nonstick saucepan, melt butter over medium-low heat.

When butter is melted, add the miniature marshmallows. Stir constantly until smooth and creamy, roughly 5 minutes.

Stir in vanilla until fully mixed.

Slowly pour the marshmallow mixture over the popcorn, turning the bowl and tossing with your hands to coat all the popcorn kernels.

Let cool one minute, then run your hands under cold water and form balls to desired size with your hands. The water keeps them from sticking to your hands. 

Shake candy sprinkles over popcorn balls if desired.

Store in an airtight container or sealed plastic bags. Or wrap in cellophane and tie with a ribbon for gift-giving.