Melt the butter, brown sugar, and corn syrup in a sauce pan and bring to a boil stirring constantly.
Allow the mixture to boil for 2 minutes and then add the baking soda, whisking well to fully incorporate. The caramel sauce will then fluff up and turn a light blonde color.
Once the baking soda has been fully mixed into the caramel sauce remove the pan from the stove and pour the sauce into the brown paper bag over all of the popped popcorn.
Roll down the top of the bag to close it and shake the bag really hard.
Place the entire bag into the microwave and microwave for 1.5 minutes.
Remove the bag from the microwave and shake again.
Microwave the entire bag a second time for 30 seconds.
After the 30 seconds remove the bag and shake one last time.
Pour the popcorn onto a parchment lined cookie sheet, and do your best to spread it out to avoid large clumps of popcorn sticking together and allow it to cool completely.
Once it is cool break apart the popcorn (it will stick a bit but is easily broken up) and enjoy!