If using slab bacon, cut into 1/2 inch dice. If using sliced bacon, leave in strips (you can crumble it later).
Cook bacon in a large frying pan over medium-low heat until cooked through, chewy not crisp (it will be in the oven for an hour with the popcorn so will crisp a bit then).
Put the cooked bacon onto paper towels to absorb excess grease.
If you used bacon slices, coarsely crumble it. Change the paper towels once or twice to make sure the bacon is not sitting in its own grease. Set aside.
Preheat the oven to 200º F.
Put the popped corn into a generously large bowl that you have spritzed with a bit of nonstick cooking spray. Make sure to remove any unpopped kernels.
Add the chopped (or crumbled) bacon to the popped corn. No need to stir it in yet. Set aside.
Cover a half-sheet baking sheet (or two smaller baking sheets) with foil and spray lightly with non-stick cooking spray. Set aside.
Into a heavy-bottomed medium pot (I used a 4 qt), combine the butter, brown sugar, kosher salt, and corn syrup. Over medium heat, stirring occasionally, bring to a boil. Boil for 5 minutes without stirring. Remove from the heat and stir in the baking soda.
Carefully pour the caramel over the popcorn and bacon and stir quickly to incorporate.
Dump the caramel-coated corn and bacon onto the baking sheet(s), spread out as evenly as possible, and place in the oven. Bake for one hour, stirring every twenty minutes, switching racks if you've used two baking sheets.
When you take the bacon caramel popcorn out of the oven, spread out on aluminum foil, parchment or waxed paper laid out on the counter to cool. It can be stored in an airtight container once all the way to room temperature and not sticky (about one hour).