Spray a 9x13 glass baking dish with cooking spray and dust with powdered sugar.
Pour 1/2 cup cold water into bowl of heavy-duty mixer fitted with whisk attachment. Gently stir in gelatin and allow to sit for 15 minutes.
Combine sugar, agave, salt, and remaining 1/2 cup cold water in heavy medium saucepan.
Cover saucepan and turn heat to medium-high. Simmer, covered for a few minutes to allow the condensation to drip down the sides of the pan, dissolving any crystals.
Remove the lid, and clip a candy thermometer to the side of the pan. Raise heat and bring syrup to a boil. Watch carefully, without stirring, until syrup reaches 240 degrees (about 8 minutes).
With mixer running at low speed, slowly pour hot syrup into gelatin mixture in thin stream down side of bowl (avoid pouring syrup onto 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 fingers. Let stand uncovered at room temperature until firm, about 4 hours.
Stir cornstarch and powdered sugar in small bowl to blend. Sprinkle over top of the marshmallows, and onto work surface.
Turn marshmallow slab out onto work surface and sprinkle more starch-sugar mixture over marshmallow slab.
Coat wheel of pizza cutter (or cookie cutters) with nonstick spray. Cut marshmallows into squares or other shapes. Toss each in remaining starch-sugar mixture to coat.
Store in an air-tight container. May be frozen for several weeks.