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. Pour the popcorn in a very large bowl.(large enough to toss with other ingredients without spilling-- I use the largest bowl I have!) (Make sure to remove any unpopped kernels before adding the popcorn to the bowl). Preheat the oven to 250 degrees F. (See Note 1)
CARAMEL SYRUP: In a pot over medium heat, add the butter, corn syrup, brown sugar, fine sea salt, and cream of tartar (not entirely necessary but helps keep the 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 it will foam up a lot! Immediately pour this mixture over the prepared popcorn in the bowl working to avoid the edges of the bowl where the caramel will harden upon contact. Working quickly, toss 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 15 minutes and rotating the pans from top rack to bottom rack. Remove the pans and allow to completely cool.
ADD MARSHMALLOWS: Once the popcorn is completely cooled, add it back to that very large bowl used earlier. Add in the miniature marshmallows and stir.
WHITE CHOCOLATE: Melt 2 and 1/2 cups (450g) white chocolate chips and 1 teaspoon 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. Pour over the popcorn/marshmallow mixture and, working quickly, stir until popcorn and marshmallows are coated in white chocolate. Pour the white-chocolate-coated popcorn back onto the lined sheet pan(s).
DARK AND MILK CHOCOLATE: Melt the dark chocolate chips with 1/2 teaspoon vegetable oil and drizzle evenly over the popcorn. (I put the melted chocolate in a small plastic bag and cut off the tip with scissors and then pipe it on.) Then, melt the milk chocolate chips with 1/2 teaspoon vegetable oil and drizzle evenly over the popcorn. Before the chocolate has a chance to harden, drop Whoppers candies evenly over the popcorn on the melted chocolate.
ENJOY: Let the popcorn harden at room temperature, about 1 hour, and then slightly break up and put in a large bowl. Enjoy!
STORAGE: To store Chocolate Popcorn, place in an airtight container (I like using Mason jars or Weck® mold jars). Keep the container in a cool dry place for up to a week (because of the marshmallows in it, we like it best within 2 days). This popcorn isn't a great candidate for freezing; the marshmallows harden when thawing and the popcorn loses texture.