Preheat oven to 225°F. Line sheet pan with parchment paper.
Place the popcorn in a roasting pan or casserole dish - something large and deep enough to give you room to stir and coat the popcorn with the caramel syrup. Put the pan in the oven so the popcorn will be warm when mixing with the caramel syrup.
40 g mushroom popcorn kernels
In a small saucepan, combine the brown sugar, butter, both corn syrups, vanilla bean paste, molasses and salt. Cook over medium low heat until the syrup reaches 250°F.
110 g brown sugar, 56 g unsalted butter, 30 ml light corn syrup, 30 ml dark corn syrup, 10 ml vanilla bean paste, 5 ml molasses, 3 g vanilla salt
Remove the pan from the heat, add the baking soda and stir quickly to combine (the mixture will fizz and bubble, you don't want to deflate the mixture so do not overmix)
1/8 tsp baking soda
Immediately pour the caramel syrup over the warmed popcorn. Use a flexible spatula to fold and stir the popcorn until it is evenly coated with the caramel syrup.
Spread the caramel popcorn out on the sheet pan. Put it in the oven for about 10 minutes, stir gently to redistribute the mixture and separate the kernels, return to the oven for 15 minutes, stir gently again and return to the oven for a final 15 minutes. Sprinkle with additional salt if desired, stir to distribute. Remove from the oven and let cool. Once cool, you can gently break up any remaining clumps. Store in an airtight container at room temperature.
Notes
If you are using US customary measurements, the 2 tbsp of popcorn kernels refers to the un-popped kernels, it would be ~4 cups of already popped popcorn.