Pop the popcorn, turn into a large pot, shake to loosen the unpopped kernels.
Transfer the popcorn to
a large pot, throw out the kernels.
Heat oven to 250 degrees. Line two pans with non-stick foil or parchment.
Dissolve
brown sugar in the butter; boil for 4 minutes.Remove from heat, add baking soda and vanilla.
Stir until glossy.
Pour
over the popcorn and mix until it's all coated.
Turn into the pans in one layer each.
Bake for 15 minutes,
stir. Repeat three more times.
