Spray two sets of molds and 4 pieces of plastic wrap generously with cooking spray and set aside.
Add 3 tbsp. of water and gelatin into a standmixer with a whisk attachment and allow gelatin to bloom.
Add sugar, corn syrup, and 1 tbsp. of water into a small saucepan and bring to a boil over medium heat and then boil hard for 1 minute.
Pour hot sugar mixture into gelatin mixture and beat on high for 12 minutes, until fluffy and stiff peaks form.
Add vanilla and beat to incorporate. Transfer warm marshmallow mixture into piping bags. Working quickly, pipe marshmallow mixture into molds. Immediately cover molds with plastic wrap sprayed with cooking spray and press down on the marshmallow filling to work the mixture into the molds.
Allow molds to sit covered a minimum of 4 hours or overnight.
Mix corn starch and powdered sugar in a shallow plate. Unmold marshmallows and toss to coat them in cornstarch and powdered sugar mixture. Store marshmallows in an airtight container.