Before starting, combine the powdered sugar with the corn starch in a small bowl. Set this aside. This will be used as a dusting powder and keep your marshmallows from being sticky.

Prepare Pan: Lightly grease a 7x11 inch baking dish and dust with confectioners' sugar and cornstarch mixture. You can also line it with parchment paper for easier removal later.

Bloom Gelatin: In the bowl of your stand mixer, sprinkle the gelatin over ½ cup of cold water. Let it sit and soften while you prepare the syrup.

Make Syrup: In a medium saucepan, combine the granulated sugar, corn syrup, salt, and the remaining ½ cup of water. Place over medium high heat, stirring until the sugar dissolves. Place a lid on the saucepan and let the the sugar mixture cook for 4 minutes.  Once the 4 minutes is up, remove the lid and increase the heat to high heat. Cook until the mixture reaches 240°F (115°C) on a candy thermometer, about 7-8 minutes.

Combine Syrup and Gelatin: With the whisk attachment on the stand mixer, turn it on low speed and slowly pour the hot syrup into the gelatin mixture in a thin stream down the side of the bowl. Be careful as the syrup is very hot.

Whip: Once all the syrup is added, increase the speed to high and whip until the mixture becomes very thick and lukewarm, about 12-15 minutes. During the last minute of whipping, add in the vanilla extract.

Pour and Set:  For best results, spray non-stick cooking spray on your spatula and scrape the marshmallow mixture into the prepared pan and spread it out evenly with a spatula. This can and will be a sticky mess! However, it is a very tasty clean up! Dust the top with more confectioners' sugar and the corn starch mixture. Let it sit at room temperature, uncovered, for at least 4 hours or overnight until set.

Cut and Store: Once set, dust a cutting board with confectioners' sugar mixture. Turn the marshmallow slab out onto the cutting board and dust the top with more confectioners' sugar. Use a sharp knife, pizza cutter, or cookie cutters to cut the marshmallows into 12 squares or desired shapes. Toss the marshmallows in more confectioners' sugar mixture to prevent sticking.

Enjoy: Homemade marshmallows can be enjoyed immediately or stored in an air-tight container at room temperature for up to 2 weeks.