Brush a 9"x13" baking dish with vegetable oil and line it with parchment paper. In the bowl of a stand mixer or a large mixing add 6 tablespoons of the water and sprinkle in both envelopes of gelatin. In a pot, add the remaining 6 tablespoons of water, granulated sugar, corn syrup, and salt. Bring the mixture to a boil over medium high heat, stirring until the sugar is dissolved.
Once the sugar is dissolved, stop stirring it and continue to cook the syrup until the it registers 238° on a kitchen thermometer. Remove the syrup off of the heat. Using the whisk attachment for the stand mixer or using a hand mixer, start to beat the gelatin mixture on low speed. Slowly drizzle in the sugar syrup with the mixer running until all of it is incorporated. Slowly raise the speed of the mixer to high and beat for 10-12 minutes until the bowl is cool to the touch and the marshmallow starts to pull away from itself. The marshmallow should be thick and shiny and should hold its shape for a few seconds when held up on the whisk.
Pour the marshmallow mixture into the prepared baking dish and use a rubber spatula to smooth it out. Set aside to allow it to set up. For softer marshmallow, allow it sit for 1-2 hours. For stiffer marshmallows that hold their shape well, allow to set for at least 3 hours or overnight for best results.
Dust a work surface with half of the powdered sugar and turn out the marshmallow onto that surface. Flip over the marshmallow so that both sides are covered with the powdered sugar.
Lightly brush a sharp knife with vegetable oil and cut the marshmallow into approximately 1 ½ inch squares. Softer marshmallows can then be rolled into balls or small logs for dipping. Stiffer marshmallows can be left in squares for dipping. Dust the pieces of shaped marshmallow with the remaining powdered sugar to prevent sticking.
Line a sheet pan with waxed or parchment paper and set aside. In a microwave safe bowl, mix together the chocolate and vegetable oil. Microwave in 30 second increments at 50% power, stirring in between, until the chocolate is 75% melted. Continue to stir until the remaining chocolate is completely melted.
Dip the marshmallow into the melted chocolate, using a fork to roll it around until all sides are covered. Place dipped marshmallows on the prepared sheet pan and cover with sprinkles.
Allow the chocolate dipped marshmallows to dry completely before storing or serving.