In a microwave-safe bowl, microwave butter, brown sugar and corn syrup on high for 1 minute. Stir and cook 2 more minutes for chewy popcorn balls. If you like crunchy popcorn balls, cook 1 minute more. Stir in vanilla and baking soda at the end.
Have popcorn ready in a large bowl. Pour caramel over the popcorn and stir to coat every piece well. Stir in candy, nuts, fruit etc. Let cool a little before handling. Using greased hands or food-safe gloves, form popcorn mixture into 2-inch balls and lay on a piece of parchment paper. (Optional: drizzle balls with melted chocolate and let it set.) Wrap each ball in plastic wrap, or keep in an airtight container for up to two weeks. Makes about 24 balls.