Add the gelatin and 1/2 cup water to a stand mixer fitted with the whisk attachment. Mix on medium speed until combined.
Add the corn syrup, granulated sugar, salt and 1/2 cup water to a medium saucepan over medium-high heat. Stir until thoroughly combined. Heat until the mixture reaches 230 degrees F on a candy thermometer, about 10 minutes.
Once the sugar mixture reaches its target temperature, SLOWLY add it to the gelatin with the mixer running on medium-low speed. Continue mixing for 10 minutes, gradually increasing the speed to medium-high. Add the vanilla and mix for another 5 minutes; the mixture should double in size and turn bright white.
Dust a plastic container (size and shape is up to you!) with sifted confectioners' sugar until the bottom is completely covered. Slowly pour the liquid marshmallows into the container and shake until evenly spread. Allow to sit, uncovered, overnight.
Before cutting, dust the top of marshmallows with confectioners' sugar; this will prevent them from sticking together if stacked.
I like to cut the marshmallows into squares for toasting. This allows for even toasting and avoids the hard, cool center that cylindrical marshmallows leave.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.