Add popcorn to a large microwave-safe bowl. Set aside.
In a medium saucepan, melt the butter over medium heat. Add brown sugar. Whisk constantly until fully combined and boiling. Remove from heat.
Whisk in corn syrup, vanilla, and baking soda.
Pour over popcorn. Fold with a rubber spatula to evenly coat.
Microwave the popcorn for 1 minute. Remove from microwave and stir. Repeat 3 more times.
Spread snack mix into a single layer onto a large baking sheet lined with foil. Allow to cool on the pan. Break apart large clusters if desired.
Cover the snack mix tightly once cooled.