Attach whisk to stand mixer. Place gelatin into mixer bowl with 1/2 cup ice cold water.

In medium saucepan, combine remaining 1/2 cup water, granulated sugar, corn syrup, and salt. Cover and cook over medium-high heat for 3-4 minutes.

Uncover pan and clip candy thermometer to side. Continue cooking until mixture reaches 240°F, approximately 7-8 minutes. Mixture will bubble, so use tall-sided pan.

Immediately remove from heat when mixture reaches 240°F.

Turn mixer on low speed. Slowly pour sugar syrup down the side of the bowl into gelatin mixture.

Once all syrup is added, increase mixer speed to high. Whip until mixture becomes very thick and lukewarm, approximately 12-15 minutes. Add vanilla in the last minute.

While mixture is whipping, combine confectioners' sugar and cornstarch in small bowl. Lightly spray 13x9 inch baking pan with nonstick spray.

Add sugar-cornstarch mixture to pan and coat bottom and sides completely. Reserve remaining mixture for later use.

Pour whipped mixture into prepared pan using lightly oiled spatula to spread evenly. Re-oil spatula as needed.

Dust top with remaining sugar-cornstarch mixture to lightly cover. Allow to sit uncovered for at least 4 hours or overnight.

Turn marshmallows out onto cutting board. Cut into 1-2 inch squares using pizza wheel dusted with confectioners' sugar mixture.

Dust all sides of each marshmallow with remaining mixture. Place in lidded container with remaining coating and shake to coat evenly.

Store in airtight container for up to 3 weeks.