Add your popcorn to a mixing bowl, making sure to remove any unpopped kernels, then set aside for later.
In a saucepan over medium heat, combine butter, brown sugar, and corn syrup. Stir until the mixture is melted and well combined.
Add in marshmallows and vanilla extract, stirring constantly until the marshmallows are melted too.
Remove from the heat and pour over your popcorn. Mix to coat evenly, then transfer to a baking sheet to set up before enjoying.
