Line 13x9x2-inch metal baking pan with foil. Coat foil lightly with nonstick spray.

Pour ½ cup cold water into bowl of heavy-duty mixer fitted with whisk attachment. Sprinkle gelatin over water. Let stand until gelatin softens and absorbs water, at least 15 minutes.

Combine 2 cups sugar, corn syrup, salt, and remaining½ cups cold water in heavy medium saucepan. Stir over medium-low heat until sugar dissolves, brushing down sides of pan with wet pastry brush. Attach candy thermometer to side of pan. Increase heat and bring syrup to boil. boil, without stirring, until syrup reaches 240°, about 8 minutes.

With mixer running at low speed, slowly pour hot syrup into gelatin mixture in thin stream sown side of bowl (avoid pouring syrup into whisk, as it may splash). Gradually increase speed to high and beat until mixture is very thick and stiff, about 15 minutes. Add vanilla and beat to blend, about 30 seconds longer.

Scrape marshmallow mixture into prepared pan. Smooth top with wet spatula. Let stand uncovered at room temperature until firm, about 4 hours.

Stir potato starch and powdered sugar in small bowl to blend. Sift generous dusting of starch-sugar mixture onto work surface, forming rectangle slightly larger than 13x9 inches. Turn marshmallow slab out onto starch-sugar mixture; peel off foil. Sift more starch-sugar mixture over marshmallow slab. Coat large sharp knife with nonstick spray. Cut marshmallows into squares or other shapes. Toss each in remaining starch-sugar mixture to coat. Transfer marshmallows to rack, shaking off excess mixture.