Special equipment: a candy thermometer; a silicone baking mat
For the cheese popcorn: Toss the popcorn with the melted butter in a large bowl until evenly coated.
In a separate bowl, combine the Cheddar powder, mustard powder and white pepper. Fold the mixture into the popcorn in two additions and toss until evenly coated.
For the caramel popcorn: Line a baking sheet with a silicone baking mat or aluminum foil and lightly spray with cooking spray.
Evenly spread the popcorn out on the baking sheet. Combine the butter, granulated sugar, brown sugar and corn syrup in a heavy-bottomed medium saucepot fitted with a candy thermometer (see Cook's Note). Bring to a boil over medium heat, stirring frequently with a heatproof rubber spatula; be careful: the caramel will bubble up a few inches as it cooks. Cook until the caramel reaches 248 to 250 degrees F (firm ball stage). Immediately turn off the heat and carefully stir in the salt and baking soda; the caramel will bubble up again when you add the baking soda.
Spray a second heatproof rubber spatula with cooking spray. Immediately, and very carefully, pour the caramel evenly over the popcorn. With the greased spatula, carefully fold the caramel into the popcorn to coat; not every kernel will be completely coated, but the caramel will be evenly distributed. Break up any large chunks with the spatula. Set aside to cool completely, at least 15 minutes.
To serve: Combine the two popcorns together, breaking up any remaining large chunks of the caramel popcorn if necessary. Serve immediately.
For the caramel popcorn, make sure to pick a medium saucepot that has at least 5-inch-high sides, as the caramel mixture will bubble up as it cooks. If your candy thermometer doesn't quite reach the caramel mixture at first, don't worry: as the caramel boils, the bottom of the thermometer will reach the mixture. Powdered Cheddar cheese comes in two versions: classic orange or white Cheddar. Using the orange Cheddar is classic to Chicago, but you can easily substitute white Cheddar powder instead. You can find powdered Cheddar cheese in most grocery stores or online. For the cheese popcorn: Powdered cheddar cheese comes in two versions, classic orange or white cheddar. Using the orange cheddar is classic to Chicago but you can easily substitute white cheddar powder instead. You can find powdered cheddar cheese in most grocery stores or online.