Lightly grease a 9x9 inch or 10x8 inch baking dish and line it with parchment paper.

In the bowl of your stand mixer, add 1/2 cup of cool water. Sprinkle the gelatin evenly on top and whisk it in. Let it sit for about 10 minutes to bloom.

In a medium saucepan, combine the remaining 1/2 cup of water, sugar, and salt. Heat over medium heat, stirring until the sugar dissolves.

Increase heat to medium-high and gently boil without stirring until it reaches 240°F.

Carefully pour the hot syrup into the bloomed gelatin while mixing on low speed. Gradually increase speed to high and beat for 8 to 10 minutes until thickened and glossy.

Add the vanilla extract and beat until just combined.

In a separate bowl, mix the powdered sugar and cornstarch.

Pour the marshmallow mixture into the prepared dish and spread evenly. Dust with cornstarch and sugar mixture.

Set aside at room temperature for at least 3 hours or overnight until firm.

Turn the marshmallow slab onto a cutting board and peel off the parchment paper. Dust the top with more cornstarch.

Cut into squares and dust again with cornstarch to prevent sticking. Store in an airtight container.