Get your popcorn all popped up and remove the kernels that are left. You could use microwave popcorn, but there is just nothing like air-popped when it comes to fluffy, amazing popcorn. I use the big roasting pan that I have "for turkeys"- and throw the popcorn in there.
Combine your butter, sugar, syrup and salt and bring to a boil on medium heat.
Now, this next part just about kills me, but DO NOT stir it for 5 minutes after it starts to boil.
Once that is done remove from the heat and add the baking soda and vanilla. The mixture will sputter and spit at you, so be careful.
Pour the caramel mixture over the popcorn and stir until coated.
Put in the oven and bake for 50 minutes to an hour. Make sure to stir it frequently, bringing the popcorn from the bottom of the roaster to the top to make sure it dries out and bakes.
Once it's done, set it aside to cool.
Once it's cooled completely, break into smaller pieces and store in plastic sealing bags at room temperature for up to a week.
Notes
Keep it stored in a cool, dry place and it will stay crispy!