Preheat the oven to 300ºF.

Grease a foil-lined cookie sheet with non-stick spray.

In a 1-gallon heavy-bottomed sauce pot, melt the butter over medium heat. Add both the corn syrup and brown sugar, stirring to combine.  Simmer until large bubbles begin to form without agitating the pan, approximately 4 minutes. Continue cooking on medium heat and stir every thirty seconds for the next 4 to 6 minutes.** Turn off the heat and carefully whisk in the salt, vanilla and baking soda. Fold in the popped corn and gently coat without crushing.  

Transfer the popcorn to the greased cookie sheet and bake at 300º F. Gently stir every 5 minutes for 15 minutes, making sure all the popcorn is evenly coated. Remove and cool the finished caramel corn on wax paper. Once cool enough to handle, but not completely cold, break apart the popcorn bunches.

*Air-popped popcorn is preferred, but stove top method will also work.

** To check the color of the caramel at this point, carefully dip one piece of the popped corn into the pot. The caramel should be amber in color.