Stir in brown sugar, corn syrup, vanilla extract, and marshmallows.
Turn heat to medium and stir constantly until marshmallows melt and mixture is smooth.
Pour hot mixture over the popcorn and stir until evenly coated. Let cool before serving.
Notes
Use freshly popped popcorn: Make sure to remove any unpopped kernels before mixing with the caramel—no one wants a surprise crunch!
Stick with salted popcorn: The salty-sweet combo really makes this treat irresistible.
Work quickly: Once the caramel mixture is ready, pour it over the popcorn right away and stir well—this helps coat everything evenly before it starts to set.
Let it cool completely: The popcorn will be sticky at first, but it firms up as it cools, making it easier to enjoy (and less messy).
Optional add-ins: Try mixing in mini chocolate chips, chopped nuts, or pretzel bits once the caramel coating cools slightly for a fun twist.