• Lightly grease a 9-inch square baking dish with nonstick cooking spray or vegetable oil.
• Pour ½ cup of water into the bowl of a stand mixer fitted with the whisk attachment. Sprinkle the gelatin evenly over the water and set it aside to bloom (hydrate) while you prepare the syrup.
• In a medium saucepan, combine: sugar, corn syrup, salt, and remaining ½ cup of water. Place the saucepan over medium heat and stir until the sugar dissolves and the mixture reaches 240°F on your candy thermometer. Immediately remove the pan from the heat.
• With the mixer running on low speed, carefully and slowly pour the hot syrup into the bowl with the bloomed gelatin. Once all the syrup is incorporated, gradually increase the mixer speed to high.
• Beat the sugar mixture on high speed until it becomes very thick and glossy, about 10 to 12 minutes.

  To Test: Stop the mixer, dip the whisk attachment into the mixture, and lift it out. If a thick ribbon forms and slowly falls back onto the mixture, it’s ready.
• Quickly beat in vanilla extract (or your preferred flavoring). Immediately transfer the sticky marshmallow mixture to the prepared pan.
• Allow the marshmallow to sit uncovered at room temperature for at least 4 hours (or overnight) to fully set.

Cut and Dust the Marshmallows

• In a small bowl, whisk together powdered sugar and cornstarch. Generously dust a clean cutting board with the sugar mixture.
• Using a sharp knife, cut the marshmallows into squares, dusting the knife between cuts to prevent sticking. Dredge each marshmallow in the sugar mixture to coat all sides and prevent them from sticking together.