Pop the microwave popcorn. Then, cut a small hole in the bottom corner of the popcorn bags and shake out the kernels.

Pour popped popcorn in a large bowl. Set aside.

Place butter, brown sugar, and marshmallows, in a sauce pan.

Bring butter mixture to a boil, stirring continually. Boil until brown sugar has dissolved and marshmallows have melted.

Remove from the heat and add the vanilla. Stir to combine.

Pour caramel over the popped popcorn and stir to distribute caramel.