Pour ice cold water into the bowl of a stand mixer fitted with a whisk attachment. Add gelatin and allow to soften, about 3 minutes.
Combine 1/3 cup water, granulated sugar, corn syrup, agave, and salt in a 2-quart saucepan. Heat over medium-high heat, stirring until sugar is dissolved. Cover; let boil 2 minutes.
Uncover saucepan. Clip candy thermometer to pan. Cook, without stirring, over medium-high heat until thermometer registers 240 degrees.
Remove from heat. Slowly pour the hot candy mixture over the gelatin mixture in the mixing bowl and stir well to combine.
Mix on low speed. Gradually increase speed to high and beat until mixture is opaque and thick, 8 to 9 minutes. Add vanilla extract.
Continue beating on high until thoroughly combined. Mixture should appear thick and form a stiff peak when whisk is removed from mixture.
Immediately transfer marshmallow mixture to sprayed and powdered sugar coated 8x8x2-inch baking dish.
Quickly and gently spread mixture evenly in pan using an oiled spatula.
Sift additional powdered sugar over the top of the marshmallow. Let stand, uncovered, 4 to 12 hours. Run an oiled knife along the edges of the pan to loosen the sides of the marshmallow mixture; invert onto large cutting board.
Using a nonstick perforated cutting wheel or buttered chef's knife, cut marshmallow into 1-1/2-to 2-inch pieces. Toss marshmallows in remaining powdered sugar.