Preheat oven to 250ºF (120ºC), use convection mode if you have it, otherwise you'll need to do two pans separately or just switch them halfway through baking.

Prepare two pans with parchment paper or a silpat and set aside.

Pop the 3/4 cups of popcorn kernels—you should have about 12 cups of popcorn. Transfer to a large mixing bowl.

In a small pan, melt butter and add brown sugar. Stir continuously and cook over medium-high heat until the mixture bubbles and rises a little bit, about 3 minutes of boiling. If you have a candy thermometer, remove the pot from heat at around 275ºF (135ºC). Stir in baking soda, vanilla extract, and if you used unsalted butter, add the salt. Stir to incorporate and then pour over the popped corn. Stir to coat, then transfer to prepared baking sheets, spreading evenly. If you're using a convection oven, bake both sheets for an hour, stirring once halfway through. If using a conventional oven, you can bake them both at the same time but halfway through, stir and switch the placements of the pans so that the top one also gets heat from the bottom.