½ tsp. canola oil ½ cup popcorn kernels 1 cup unsalted butter, plus more for greasing 2 cups light brown sugar ½ cup light corn syrup ¼ tsp. cream of tartar 1 tsp. sea salt, plus more to taste 1 tsp. vanilla extract
Heat oven to 250°. Heat oil in an 8-qt. saucepan over high heat. Add popcorn, cover and cook, shaking pan, until popping subsides, 6–8 minutes. Let sit 1 minute. Grease a foil-lined baking sheet; cover with popcorn. Combine butter, sugar, syrup, tartar, and salt in a 2-qt. saucepan fitted with a candy thermometer over high heat. Cook, without stirring, until mixture is a dark brown and registers 260°. Remove from heat; stir in vanilla. Pour over popcorn and mix; spread in an even layer. Bake until caramel is soft again, 25–30 minutes. Stir to coat popcorn, season with salt; cool.