Pop at least 12 cups of popcorn in an air popper or the microwave. DO NOT add salt or butter! Place it in a bowl large enough to stir in the syrup mixture for coating or place it on a parchment paper-lined rimmed baking sheet. You may also put it in a prepared baking dish large enough to hold the 12 cups of popcorn and mix it all up from there.
In a small saucepan, melt butter with the brown( light or dark) sugar and dark corn syrup over LOW heat. Boil for 5 minutes, stirring occasionally; remove from the heat and whisk in the baking soda, salt and vanilla. It will become foamy.
Drizzle over the popcorn and stir to coat as evenly as possible.
Bake at 250°F for 45 minutes, stirring at 15 minute intervals.
Remove from the oven and allow to cool slightly. Break up any big clumps and store in sealed container or large Ziploc bags.