Prepare a 9x13 dish with grease and set aside.
In a large bowl or in the bowl of a stand mixer or large bowl, bloom the gelatin. Add the gelatin with ½ cup of water and stir until combined. Set aside to bloom.
In a deep saucepan, combine the remaining water, sugar, corn syrup, and salt. Mix on a medium heat until the sugar is dissolved.
Once the mixture is combined, turn the heat up to medium-high. Let the sugar mixture cook undisturbed until the temperature reaches 240°F. Resist the urge to stir the mixture at any point.
Next, add the sugar mixture to the gelatin. You will want to do this slowly. With the stand mixer on low, or with the hand mixture ready to go, add the sugar mixture. Slowly begin beating the mixture. Once the mixture is fully in the bowl, increase the speed. The mixture will look a little pale in color, but that's good. Whip the mixture until it is very fluffy and white in color. This will take anywhere from 7-12 minutes, depending on the speed of your mixer. Mix in the vanilla at the end.
Once the mixture is super fluffy and cool, spread it into the prepared dish. If you wet your spatula with water it will be much easier to spread it. Once in the pan, top the fluff mixture with confectioners sugar.
Make the chocolate mixture. In a double boiler, melt the chocolate. Once melted stir in the milk until you reach your desired thickness.
Add small dollops of the chocolate mixture into the pan with the marshmallows and swirl. Use a toothpick or a fork in order to get the maximized amount of swirls.
Let the mixture sit for a few hours at room temperature or in the refrigerator overnight.
The marshmallows should spring back when ready. They might be a little bit sticky when cutting, but a cold knife will help reduce that.
Store in an air-tight container or in the freezer.