1. Put popped popcorn in a paper bag, set aside. Place stick of butter, brown sugar, salt and corn syrup into microwave safe bowl.
2. Place in microwave and heat until it bubbles. Times will vary by microwave so you will want to watch it closely. Once melted and bubbly, remove from the microwave and stir.
3. Return to microwave and cook for 1 minute. Remove again, add baking soda and stir well. Mixture will thicken and lighten in color. Pour mixture over the popcorn inside the paper bag.
4. Fold the top of the paper bag down several times to close- then shake really well to distribute all of the caramel over the popcorn.
5. Place entire bag of popcorn back into the microwave and heat for another 1 minute. Remove and shake some more.