Line an 8x8 in (20x20 cm) pan with parchment paper. Set aside.

Pour 1/2 cup (118 mL) warm water into a large bowl (it will barely cover the bottom of the bowl). Sprinkle gelatin over the water and whisk immediately. Set aside.

Meanwhile, add remaining 1/2 cup (118 mL) water, powdered sweetener, and sea salt to a large saucepan. Heat over low to medium heat for a few minutes, stirring frequently, until the mixture is hot, but not boiling, and sweetener dissolves. (The color will change from opaque to slightly translucent, and remove immediately as soon as you see bubbles starting to form at the edges.)

Remove from heat. Stir in vanilla extract. Pour the hot liquid into the large bowl with gelatin, while whisking constantly.

Using a hand mixer on high power, beat the mixture for about 12-15 minutes, until the volume doubles and the mixture looks very fluffy, like stiff egg white peaks. (The time could take longer depending on the size of your bowl and how powerful your mixer is.)

Transfer the marshmallow mixture into the prepared pan.

Refrigerate for at least 8 hours, or overnight, until firm and no longer sticky. Use a sharp chef's knife to cut into squares.