Heat oven to 300°F. Line a rimmed baking sheet with parchment paper; set aside. Lightly coat parchment and a very large bowl with nonstick spray.

Place 2 tablespoons oil and 2 to 3 kernels in a 3-quart or larger pot. Turn heat to medium-high and cover with a lid. When you hear these first kernels pop, add the remaining kernels and replace the lid. Using potholders, shimmy the pot around to keep the kernels moving as they pop. When several seconds pass between pops, remove from heat. Transfer to coated bowl. You should have about 8 cups popcorn.

Wipe out the pot you used to pop corn. Add sugar and water to it, and bring to a boil over medium-high heat, only stirring until sugar has dissolved. Boil, swirling the pan around (wear potholders) until caramel is a deep amber color, which should take about 8 to 9 minutes in an already-heated-from-the-popcorn pot. Remove from heat and stir in butter (mixture will hiss and bubble furiously). Return to stove, reduce heat to medium-low, and cook for another 2 to 3 minutes, removing from the stove if it appears smoky sooner. Remove from heat and whisk in the salt, baking soda, and miso.

Quickly pour the caramel over the prepared popcorn and toss to coat using two silicone spatulas.

Spread popcorn on the prepared baking sheet. Sprinkle sesame seeds over. Bake in oven, tossing occasionally until the mixture is dry, about 15 to 20 minutes. Let cool completely on the baking sheet, then break it up into a large bowl.