Prepare Marshmallows through melting the gelatin. Have the corn syrup, water, and salt ready, and increase the salt to ½ teaspoon. Place the 2 cups sugar in a large heavy saucepan and melt over medium heat. Allow the sugar to caramelize, swirling the pan to dissipate any hot spots, until very dark (you want the caramel to be on the ragged edge between very dark caramel and burnt sugar). Very carefully (there will be lots of hot steam!), pour the water, corn syrup, and salt into the caramelized sugar. The caramel may seize up. This is okay—simply remove it from the heat and stir until the caramel re-melts. Return the pan to the heat, increase to medium-high, and boil the syrup until it reaches 245°F, the firm-ball stage. Proceed as directed.