Mix the sugar, salt, and cinnamon together in a small bowl and set aside. Place a rimmed baking sheet or a large paper bag on the work surface.
Heat the coconut oil in a large heavy-bottomed pot or Dutch oven over medium heat for 1 minute. Add 2 popcorn kernels. Once they pop, add the scraped vanilla seeds and stir. Immediately stir in the remaining popcorn kernels, wait 10 seconds, then quickly stir in the sugar-cinnamon mixture. Cover the pot, leaving the lid slightly askew (and facing away from you) to allow steam to escape. Use oven mitts to protect your hands as you shake the pot constantly over the burner. Keep shaking until most of the popcorn has popped and you hear 1 to 2 seconds between pops.
Immediately turn off the heat, uncover the pot, and carefully dump the popcorn out onto the baking sheet or into the paper bag. Let cool at least 5 minutes before eating (be careful-the sugar makes the popcorn extremely and hot!).
Recipe courtesy of Melissa d'Arabian