In a medium saucepan, combine the butter, brown sugar, corn syrup, and vanilla extract and heat over medium heat until the butter is melted and the mixture is bubbling.
Add the baking soda and stir until combined. The mixture will foam up and become slightly thicker.
Add the popcorn to a large bowl and pour the caramel mixture over the popcorn. Stir to combine.
Add the marshmallows and stir until evenly distributed.
Pour the popcorn onto a parchment-lined baking sheet and spread out into an even layer.
Bake at 350°F for 8-10 minutes, stirring once during baking.
Remove from the oven and let cool completely before serving.
Interesting Facts
Popcorn is a type of corn that “pops” when heated.
Marshmallows have been around since Ancient Egypt.
Caramel popcorn is a popular snack in the United States.