Set 2 large bowls in a work area.
Preheat oven to 250 degrees F.
In a large pot, add popcorn cooking oil and heat over medium-high heat.
Add 3 popcorn kernels and continue heating over high heat.
After all 3 kernels pop, add the remaining popcorn kernels and cover tightly with a lid.
Using potholders, grab the handles of the pot and the lid and shake pot occasionally to move unpopped
kernels to the bottom of the pot. If the popcorn begins to push against the lid, carefully shake some into a large bowl, replace the lid and continue cooking until most kernels are popped.
One popping has stopped, turn off the heat and dump the popcorn into a large bowl.
Carefully transfer popcorn a few handfuls at a time into the second large bowl. Watch for unpopped kernels and remove. They will break the teeth of unsuspecting victims. Discard unpopped kernels.
Set popcorn aside.
Spray a large roasting pan, or 3 - 9X13 cake pan with baking spray and set aside.
In a medium saucepan, melt butter over medium heat.
When butter is melted, add brown sugar and corn syrup, mix well.
Continue heating over medium heat, stirring frequently until the mixture comes to a boil.
Continue boiling for 5 minutes.
Remove pan from the heat and add baking soda, stirring well. The mixture will puff up some, just keep stirring until there is no visible baking soda.
Pour hot caramel mixture over popcorn and stir well, coating all the pieces of popcorn as best you can. The caramel will continue to spread out and coat more spots with caramel during the next cooking step.
Transfer popcorn to baking pan/pans.
Spread out evenly.
Place in preheated oven and cook for 45 to 60 minutes, stirring every 15 minutes to help coat the popcorn and rotate pieces so that there are no burnt pieces. Remove popcorn from the oven just as it starts to darken slightly.
Pour onto foil sprayed with baking spray or wax paper.
Popcorn may be clumpy, as it cools just break it apart.
Store in an airtight container.