Pop popcorn, and place in a large roasting pan. Mix in any desired additions (cereal, pretzels, nuts, etc).
In a medium saucepan, add butter, brown sugar, salt and corn syrup.
Cook over medium-high heat (not high), stiring occasionally until the mixture begins to boil.
Boil 5 minutes, or until the mixture is a deep amber color, stirring occasionally. Set a timer, and do not leave the kitchen. The mixture can burn quickly. Keep the stove temperature just high enough to keep the mixture boiling, and no more (this may be medium, or medium-high). If you prefer to use a thermometer, the mixture should be about 234*F, or soft ball stage.