Lay a piece of wax or parchment paper on the counter or on a baking sheet. Place popped popcorn in a large bowl. Set aside.
Place butter, brown sugar and corn syrup in a microwave safe bowl. Cook for one minute. Stir and cook two additional minutes for chewy popcorn balls. If you like crunchy popcorn balls, cook an additional minute. Remove from microwave. Stir in vanilla and baking soda.
Pour caramel over popcorn and stir until well coated. Allow to cool slightly.
Grease hands with butter (or use wax paper or gloves) and shape the popcorn into 2-inch balls. Wrap each ball with plastic wrap or store in a zip top bag or airtight container for up to 2 weeks.
For a fun twist, mix in m&m's, candy corn and/or nuts.
One batch goes a long way. It will typically make 24 (2-inch balls) , but depending on the size could make more or less.
Tip: To test if your caramel has reached a soft ball, grab a cup of ice water. Then dip a spoon in the caramel and then dip it into the ice water. It should form a soft ball (chewy caramel). If it's still stringy, cook an additional minute.