Preheat oven to 250°F (120°C). Line a baking sheet with parchment paper.
In a large saucepan, combine brown sugar, butter, and corn syrup. Cook over medium heat, stirring constantly, until butter is melted and sugar is dissolved. Bring to a boil and cook for 5 minutes, stirring occasionally.
Remove from heat and stir in vanilla extract and salt.
In a large bowl, combine popped popcorn and marshmallows. Pour the caramel mixture over the popcorn and marshmallows. Stir gently to coat evenly.
Spread the mixture in a single layer on the prepared baking sheet.
Bake for 25-30 minutes, stirring every 10 minutes, until golden brown and slightly crisp.
Let cool completely on the baking sheet before breaking apart.
Notes
Store in an airtight container at room temperature for up to 3 days.