Brush a 9"x13" glass baking dish with vegetable oil and line with parchment paper. Set aside.
In the bowl of a stand mixer add 6 tablespoons of water and sprinkle in both envelopes of gelatin. Set aside.
In a saucepan, add the remaining 6 tablespoons of water, granulated sugar, corn syrup, and salt. Bring mixture to a boil over medium-high heat, stirring until the sugar is dissolved. Once sugar is dissolved, stop stirring and continue to cook until the mixture registers 238° on a candy thermometer (approximately 8-10 min). Remove from heat.
Take the bowl of water and gelatin and attach it to a stand mixer fitted with a whisk attachment. Turn mixer on low and while it is mixing, slowly pour in boiled sugar syrup. Once the syrup is completely added, slowly raise the speed to high on the mixer. Beat the mixture on high until it becomes stiff (approximately 10-12 minutes). Beat in vanilla.
Pour marshmallow mixture into greased glass baking dish and use a spatula to smooth it out. Set aside to cool completely and set up (approximately 3 hours).
Once the marshmallow is cooled and set up, lightly dust a work surface with ½ cup powdered sugar and unmold onto the surface. Flip it over so both sides are covered with powdered sugar. Lightly brush a sharp knife with vegetable oil and cut the marshmallow into approximately 1 ½" squares. Dust the squares with the remaining ½ cup of powdered sugar.
Line a sheet pan with waxed or parchment paper and set aside.
In a microwave-safe glass bowl, add chopped chocolate and 2 teaspoons of vegetable oil. Microwave in 30-second increments at 50% power, stirring in between. Be careful not to microwave too long or the chocolate will burn. Only cook it until the chocolate is 75% melted and then stir it until it finishes melting.
Dip marshmallow squares into the melted chocolate, using a fork to roll it around until all sides are covered. Place dipped marshmallows on the prepared sheet pan to dry. Sprinkle tops of the dipped marshmallows with sprinkles if desired.
Let chocolate set up completely before serving or storing.