Combine the gelatin and ½ water into the bowl of a stand mixer and let sit.
Combine remaining ½ cup water, sugar, corn syrup, and salt in a small saucepan. Place over Medium-High heat, cover and cook for 3 to 4 minutes.
Uncover and clip a candy thermometer to the side of the pan a continue to cook until temp reaches 240°F. Should take about 7 to 8 minutes. Once the mixture reaches this temperature, immediately remove from heat.
Turn the mixer on low speed, while running, slowly pour sugar mixture down the side of the bowl into the gelatin mixture.
Once you have added all the sugar syrup, turn speed to high and whip until the mixture becomes very thick and lukewarm, about 15 minutes. Add the vanilla during the last minute of whipping.
Combine confectioner’s sugar and corn starch together in a medium bowl.
Lightly spray a 9x13-inch baking pan with cooking spray and dust with the powdered sugar mixture.
Spread marshmallow cream out in the pan so it’s relatively level. Sprinkle with more powdered sugar mixture. Let sit uncovered for at least 4 hours, preferably overnight.
Once set, turn marshmallow out onto a cutting board and slice into 1-inch squares with a pizza cutter dusted in powdered sugar mixture.
Once cut, dust every side of each marshmallow with powdered sugar mixture, make more mixture if needed.
Store in an airtight container for up to 3 weeks!