In a medium-sized pot, combine the brown sugar, agave. oat milk, butter, vanilla and salt, Stir and bring to a boil over medium heat. Once boiling, stir in the baking soda and reduce heat. Stir well as the mixture foams. Cook for 2 minutes on low and then pour the hot mixture over the popcorn. Stir quickly and well to cover as much of the popcorn surface as possible. If you are adding nuts, this is the time. Make sure the nuts get coated as well!