Microwave butter, sugar, syrup and salt on high for 2-2 1/2 minutes, stirring once after the first minute.
After complete cycle, stir in the baking soda. Mixture will get cloudy and creamy looking.
In a large brown bag, pour 4 quarts popped popcorn with nuts, if desired. Pour mixture over popcorn and stir to coat.
Fold top of bag over and microwave on high for 1 1/2 minutes.
Stir contents, return to microwave and cook another 1/2 minute on high.
Pour onto cookie sheet or waxed paper for caramel corn to cool faster.
When completely cooled, break apart.