Preheat oven to 250 degrees. Spray two large, rimmed baking sheet pans with non-stick spray.
Add popcorn and peanuts to a large, oven-proof mixing bowl that has been coated with non-stick spray. (I use a large stainless steel bowl.) Place in oven to warm while making syrup.
In a heavy 2-quart saucepan, combine butter, brown sugar, corn syrup, salt, and molasses. Heat over medium heat, stirring until sugar is dissolved. Continue to boil, stirring often, until the mixture reaches a hardball stage of 260 degrees on a candy thermometer (about 5 minutes).
Remove from heat and whisk in the baking soda. Mixture may foam a little.
Immediately pour the hot mixture, in a thin stream, over the warm popcorn. Using a rubber spatula, gently fold the caramel into the popcorn until all of the popcorn is coated.
Transfer the mixture to the prepared sheet pans, spreading it out to cover pans.
Bake for 45 minutes, stirring the caramel from bottom of pan and into the mixture every 15 minutes. Cool and serve immediately, or store in an airtight container for up to 1 week.