Position oven racks in upper and lower third of.  Preheat oven to 250°F/120°C. Line two rimmed baking sheets with aluminum foil or parchment paper, coat lightly with nonstick spray; set aside.

Lightly coat a very large bowl with nonstick spray. Place popped corn and cereal in the bowl. Lightly coat a large spoon or rubber spatula with nonstick spray; set aside.

Place the butter, brown sugar, corn syrup and salt in a heavy-bottomed medium-sized saucepan and stir to combine. Cook over medium-high heat until it boils, then boil for about 4 minutes without stirring. If you have a candy thermometer, you want the mixture to come to 275°F/135°C. Remove from the heat, add the vanilla and baking soda and swirl the pot to incorporate. Immediately and slowly pour the caramel over the popcorn and cereal, stirring and tossing it constantly with the prepared spoon/spatula. You want to coat the popcorn/cereal as evenly as possible with the caramel. Taste the popcorn at this stage. If you want more salt, sprinkle some additional salt over the popcorn as you toss it. If you like it hot, sprinkle with cayenne to taste.

Divide the caramel-coated popcorn/cereal between the two baking pans, breaking up any large clumps. Bake for 1 hour, tossing two or three times during the baking time. Cool completely on racks. Popcorn is ready to serve. Store at room temperature in airtight containers for up to 1 week.