Whisk the first 2 ingredients together in a bowl and set aside. Spray whatever pan you intend to use to hold the marshmallows with your nonstick spray.
Next, sift some of the cornstarch/sugar mixture into the pan and shake around, tipping it to the sides as well to coat the whole thing and keep the marshmallows from sticking while they set. Keep any leftover mix.
Now, pour 1/2c (118.5ml) ice water into the bowl of your stand mixer and add the gelatin. Do not stir. Just let them sit for 5 minutes.
While the gelatin is hydrating, add the sugar, corn syrup, salt, and remaining water to a medium sauce pot and give it a quick stir to combine. Turn the burner to medium-high heat and let the mixture heat up until most of the sugar is melted and the syrup begins to bubble. (Do NOT stir the mixture once the heat is on. Too much agitation by outside objects could make the syrup seize)
Turn the heat higher a bit at a time to bring the syrup to 240F (115.5C) Once reached, immediately move the pan from the burner.
Switch on the stand mixer to the lowest setting and very slowly pour the SUPER HOT syrup into the mixer. When combined, you can begin turning up your mixer to high. Let your mixer run for 10-15 minutes. As the mixer incorporates air into the syrup it will increase in volume and turn white, like marshmallow fluff.
When it hits this consistency add the vanilla and whisk for about 1 minute more.
Pour the cooled ganache into the marshmallow mixture and very gently fold it 2-4 times with a spatula that has been sprayed with nonstick spray. You don't want to incorporate the ganache, just create some marbleization.
Using the spatula, work quickly to scrape and spread the marshmallows evenly into your powdered and prepared pan(s). If the mixture starts sticking to the spatula, give it another hit of spray.
Let the marshmallows set on your counter for a minimum of 4 hours before turning out onto a cutting board to slice them up. After cutting, roll each marshmallow in the leftover cornstarch-sugar mixture to keep them from sticking to each other.