PREP: Assemble all the ingredients. Line one extra-large (15x21-inch) or 2 large cookie sheets with parchment paper or a Silpat liner and set aside. Remove any unpopped kernels. Pour the popcorn in a very large bowl (large enough to toss with other ingredients without spilling-- I use the largest bowl I have!). Preheat the oven to 200 degrees F.
CARAMEL SYRUP: In a large pot over medium heat, add the butter, corn syrup, brown sugar, fine sea salt, and cream of tartar (optional but helps to keep popcorn from crystallizing). Stir constantly until the butter is melted and then bring to a boil. Let boil for 5 minutes (still over medium heat) without stirring. Remove from the heat and stir in the baking soda and vanilla extract. Watch out-- the vanilla may sputter a bit and the mixture will foam up a bit! Immediately pour this mixture over the prepared popcorn, working to avoid the edges of the bowl where the caramel will harden on to upon contact. Working as quickly as possible, toss popcorn with a wooden spoon to combine and cover the popcorn in the caramel syrup mixture.
BAKE: Spread the coated popcorn on the prepared cookie sheet(s) in one even layer. Bake for 1 hour, stirring every 20 minutes with a spatula and then spacing the popcorn again in one even layer. Remove the pan(s) and allow to completely cool.
CINNAMON-SUGAR: Combine the white sugar and cinnamon in a small bowl. Stir and set aside.
WHITE CHOCOLATE: Melt white chocolate chips and vegetable oil in a microwave-safe bowl in bursts of 30 seconds, stirring in between each burst for 15 seconds until the chocolate is melted and smooth. Transfer melted white chocolate to a plastic bag and seal the bag without air in it. Cut the tip off of the bag and pipe this melted white chocolate over the popcorn. (It may seem like too much, but we want the popcorn well coated!) As soon as the popcorn is covered in white chocolate, sprinkle the cinnamon-sugar mixture evenly over everything.
ENJOY: Let the popcorn harden at room temperature, about an hour, and then break up slightly and put in a large bowl. Enjoy!