Pour 1/2 cup of hot water into the bowl of an electric stand mixer and sprinkle with the gelatin. Let stand for about 10 minutes.
In a medium saucepan, combine the sugar, corn syrup and remaining water, stirring until sugar is dissolved. Bring to a boil and cook, without stirring, until a cooking thermometer registers 240 degrees. Keep a close eye on the mixture once the thermometer hits 200 degrees, as it will burn if the mixture gets much hotter than 240.
Remove the mixture from heat and pour over the gelatin mixture, stirring until gelatin is dissolved.
With a whisk attachment, heat the mixture on high until pale white and nearly tripled in volume, about 6 minutes.
In a separate large bowl, beat the egg whites until they just hold stiff peaks. Add the whites, caramel sauce, bourbon and 1 tsp. vanilla to the mixture and mix until just combined.
Pour the mixture into a 9×13″ pan that is oiled and generously dusted with powdered sugar (bottom and sides). Sift a thin layer of powdered sugar evenly over top. Let sit uncovered at least three hours, up to one day.
To unmold, run a thin knife around the edges of pan and invert onto a large cutting board. Lifting up 1 corner of the inverted pan, use your fingers to loosen marshmallow and let drop onto cutting board. Either trim edges of marshmallow and cut into cubes or use a cookie cutter to make shaped marshmallows.
Toss the marshmallows in a large bowl of sifted powdered sugar. They will keep in an airtight container for up to 1 week. I find that putting extra powdered sugar in the container helps them stay fresher.
For the salted caramel sauce: Heat the sugar in a saucepan over medium heat until the sugar dissolves and starts to turn amber colored. Turn heat to low, add the butter and stir until melted. Then add the cream, vanilla and salt, stirring to combine. Continue stirring for about 5-10 minutes, until it comes together. Let come to room temp before adding the to marshmallows. Store the extra in the fridge and use on ice cream or eat by the spoonful! 🙂