Combine butter, brown sugar, corn syrup and salt in pan and heat over medium heat, stirring until dissolved.
Cook to firm ball stage, 248°F. (on a candy thermometer)
Add baking soda (will foam!)
Pour mix over popped corn and mix well.
Bake at 250°F for about 90 minutes, stirring every 15 minutes or so.
Cool and enjoy!
I like to double the caramel sauce recipe for more coverage on the popcorn.
Can also add salted peanuts if desired.