In the bowl of a stand mixer with a whisk attachment, add the gelatin. Pour ½ cup of the cold water over the gelatin and let it sit a few minutes.
Meanwhile, in a small saucepan over medium-high heat, pour in the remaining cold water, sugar, corn syrup, and salt. Cover and cook for 3-4 minutes, or until the sugar begins to dissolve.
After 4 minutes, uncover the saucepan and fit with a candy thermometer. Continue to cook until the thermometer reads 240°F (7-8 minutes). Remove from heat. (If you don't have a candy thermometer, bring the mixture to a rolling boil and let it boil for one minute before removing from heat.)
With the mixer on low speed, carefully pour the sugar mixture into the gelatin mixture. After all the sugar mixture has been added, increase the speed to high. Continue to whisk on high for 10-15 minutes, or until the mixture is very thick. At the last minute, add the vanilla. (For a stiffer marshmallow, whisk the full 15 minutes.)
When ready, pour the marshmallow mixture on top of the slightly cooled brownies, spreading it out evenly. Make sure your tools are sprayed with non-stick spray as well!
Let the brownies sit out for at least 4 hours, up to overnight.
When ready, preheat your oven to broil.
Place the brownies under the broiler (on the top rack) for 5-10 seconds, or until they are nicely toasted. This happens quickly so don't walk away! You can also use a blowtorch if you have one (and know what you are doing!).
Allow the brownies to come back to room temperature and then top with chocolate syrup (optional).
The marshmallow brownies keep well in a closed container in a cool dry space for 3 -4 days.