Heat the oven to 350°F and have a large sheet pan lined with parchment paper nearby.
In a large, heavy-bottomed pot, combine the sugar, butter, corn syrup, salt and 1 tablespoon water.
Cook over medium-high heat, stirring with a large metal spoon or rubber spatula, until the butter melts and the mixture turns into thick bubbling syrup; about 1 minute.
Cook until it begins to smell toasty; about 1 more minute; then add the popcorn, carefully tossing every 2 to 3 seconds, until it starts to brown lightly; another 1 to 2 minutes.
Begin tossing constantly for another minute or two until the popcorn turns deep golden brown in most spots, being careful to never let it burn.
Remove from the heat, sprinkle with the cinnamon and toss a final time to coat.
Immediately pour the toasted popcorn onto the lined sheet pan and spread in an even layer.
Place in oven and bake until a shade darker; 8 to 10 minutes.
Remove from oven; cool the popcorn on the pan, on a baking rack, for 5 to 10 minutes to harden.
Toss a final time to break up any clumps and serve.
