Pop 16 cups of popcorn. Spread out on baking sheets to remove any unpopped kernels, then return to large bowl.
Preheat oven to 200º F and grease the 2 baking sheets.
In a large pot combine butter, sugars and corn syrup. Bring to a boil on medium high heat and stir until sugars have dissolved. Turn down heat and continue to boil for 5-6 minutes, until caramel reaches a hard ball stage or 250 F. You can test the caramel by dripping a little into a bowl filled with ice cold water. If the caramel forms a hard ball, you are good to go!
Turn off heat and add in remaining ingredients and stir until well combined. Caramel will bubble up when the baking soda is added.
Pour finished caramel over popped popcorn in a large bowl and stir quickly to cover all of the popcorn. Pour out caramel popcorn onto 2 greased baking sheets and spread out evenly. Bake in 200º F oven for 1 hour.
Remove the popcorn from the oven and break up any large pieces. Store in an airtight container to keep fresh.