Marshmallows will set up overnight after cooking and mixing. Each recipe will make 24-36 marshmallows.
In the bowl of an electric mixer fitted with the whisk attachment, combine the gelatin and 1/2 cup of cold water and allow the mixture to sit while you make the syrup.
In a small saucepan over medium heat, combine the sugar, corn syrup, salt, and 1/2 cup water. Cook until the sugar dissolves. Raise the heat to high and cook until the syrup reaches 240℉ degrees on a candy thermometer. Remove the syrup from the heat.
Turn the mixer on low speed and slowly add the sugar syrup into the dissolved gelatin. Increase the speed of the mixer to high and whip until the mixture is very thick, about 15 minutes. Add the vanilla and mix thoroughly.
Using a sieve, generously dust an 8 x 12-inch nonmetal baking dish with powdered sugar. Pour the marshmallow mixture into the pan, smooth the top, and dust with more powdered sugar. Allow the marshmallows to stand uncovered overnight until they dry out.
Turn the marshmallows onto a board and cut them in squares. Dust them with more powdered sugar.
Add the unsweetened cocoa powder to the sugar syrup before beating.
Dust the pan and the marshmallows with powdered sugar mixed with cocoa powder.
Add the salted caramel syrup to the sugar syrup before beating.
Optional--Before serving, drizzle with a small amount of additional salted caramel syrup.