Line a large baking sheet with parchment paper or aluminum foil and grease generously with the spray. Generously spray a large bowl and all but the handle part of a large spoon or rubber spatula.
Combine the popcorn, salt and oil in a large deep pot with a lid and stir to coat the kernels in oil. Place the pot over moderately high heat, cover with a lid and cook, shaking the pot frequently to redistribute the kernels. Cook until all the kernels have popped, 6 to 8 minutes. Transfer the popcorn to the large prepared bowl, discarding any un-popped kernels.
Combine 1/4 cup water, the sugar and corn syrup in a large deep pot over moderately high heat and stir to combine. Cook, undisturbed, until the mixture develops a light amber color, about 10 minutes. Remove the pot from the heat, add the butter and baking soda and stir continuously to melt the butter and combine the ingredients, about 20 seconds. Working quickly, pour the sugar mixture (caramel) over the popcorn, and add the almonds, cashews and chocolate, if using. Use the prepared spoon to stir everything together, trying to evenly coat the popcorn and nuts in caramel. Sprinkle with sea salt. Break the caramel corn into smaller, bite-size pieces and serve.
Recipe courtesy of Michelle Boyd, Sugar Dome