Spray a brown paper bag with cooking spray. Add popped popcorn.
Measure out baking soda; set aside.
Combine butter, sugar, corn syrup and salt in a saucepan; bring to a hard boil, stirring constantly. Boil for 2 minutes (still stirring), then remove from heat and immediately stir in baking soda.
Pour mixture over popcorn and stir to combine. Once popcorn is fairly evenly coated with caramel mixture, fold the top of the bag down and shake. Put bag in the microwave and cook on high for 1 minute. Take the bag out and shake again (bag will be hot!). Return to the microwave and cook for 1 minute more. Take out, put in a bowl and EAT!
Caramel corn will harden and start to stick together as it cools, so you may want to stir it to break it apart periodically. This is easiest to do with the handle of a wooden spoon.
Notes
If you have a high-powered microwave, you may want to lower it one or two levels, or slightly adjust the microwave time, to keep the sugar in the caramel from burning.If you want a darker color on your caramel corn, you can try microwaving it for a few seconds longer. Do undertake this at your own risk, however, as the caramel can burn very quickly.