Lightly grease a 9×9 or 10×8-inch baking dish and line it with parchment paper.

In a stand mixer bowl with a whisk attachment or a large mixing bowl with a hand mixer, pour in ½ cup of cool water. Sprinkle the gelatin evenly over the top and whisk. Let it sit for about 10 minutes to bloom (soften and absorb the water).

In a medium saucepan, combine the sugar, salt, and remaining ½ cup of water. Heat over medium heat until the sugar dissolves, then increase to medium-high.

Bring to a gentle boil, without stirring, until it reaches 240°F (known as the 'soft ball stage') on a candy thermometer.

Once the syrup reaches temperature, carefully pour it into the bloomed gelatin while mixing on low speed. Gradually increase to high speed and beat for 8 – 10 minutes until the mixture becomes thick, glossy, and tripled in volume.

Add the vanilla extract and beat just until combined.

In a separate bowl, combine cornstarch and powdered sugar.

Quickly pour the marshmallow mixture into your prepared pan, spreading it evenly with a greased spatula. Dust the top with the cornstarch and sugar mixture, then let it set at room temperature for at least 3 hours, or overnight, until firm.

Once set, turn it out onto a cutting board, peel off the parchment, dust more cornstarch mixture over the top, and cut into squares using a sharp knife or kitchen scissors.

Place in a bowl and dust again, tossing to coat, to prevent the marshmallows from sticking. Store in an airtight container at room temperature.