Pop popcorn and pour into electric turkey roaster or very large oven roasting pan.
Heat the the roaster or your oven to 200 degrees.
Making the caramel syrup: in a saucepan combine the butter, brown sugar, corn syrup and salt.
Boil for 5 minutes and remove from heat; add the baking soda and stir well.
Pour over popcorn and stir to combine as good as possible.
Bake for 2 hours stirring every 15 minutes.
Pour out on wax paper to dry.