Place 1/3 cup popcorn kernels into a brown paper bag. Fold the top of the bag over twice to seal, then microwave until the popping slows and the popcorn is done.
Repeat with the remaining 1/3 cup of kernels.
Transfer all the popcorn to a large bowl. Shake the bowl so any unpopped kernels fall to the bottom, then measure out 15 cups of popped popcorn into a second large bowl. This separation step is important, so no hard kernels end up in the finished caramel corn. Set aside.
In a large pot on the stovetop, melt the butter, then add the Karo corn syrup, light brown sugar, and salt. Stir until combined. Increase the heat and bring the mixture to a boil. Let it boil for 1 minute, then remove from the heat.
Stir in 1 teaspoon of baking soda. The caramel will foam up and expand.
Carefully pour the hot caramel over the popcorn. Use a spatula to toss and coat the popcorn evenly.
Spread the caramel corn onto a large baking sheet to serve. It’s especially delicious when it’s warm, so give it a try!
Once completely cooled, store in gallon-size zip-lock bags to keep it soft for several days.