makes about 4 dozen large marshmallows
3 packages unflavored gelatin
1 cup ice cold water, divided
1 1/2 cups granulated sugar
1 cup light corn syrup
1/4 teaspoon fine sea salt
1 teaspoon vanilla extract
1/4 cup powdered' sugar
1/4 cup cornstarch
1. Add the gelatin to the bowl of an electric mixer along with 1/2 cup of ice cold water.
2. In a small saucepan, add together the remaining 1/2 cup of ice cold water, granulated sugar, corn syrup, and salt. Turn the heat to medium high, cover, and let cook for 3-4 minutes. Remove the lid and clip a candy thermometer onto the side of the pan and continue cooking until the mixture reaches 240 degrees F, 8-15 minutes. Once the mixture reaches 240 degrees, remove immediately from the heat.
3. With the whisk attached, turn the mixer on low speed, and while running, slowly (and carefully!) pour the sugar syrup down the side of the bowl. Once all of the syrup has been added, increase the speed to high and whip until very thick, about 15 minutes. Add the vanilla during the last minute of whipping.
4. While the mixture is whipping, prepare the pan: spray a 9" x 13" metal baking pan with nonstick spray. In a small bowl, sift together the powdered sugar and cornstarch. Sift some of the powdered sugar mixture onto the bottom of the pan, then carefully shake to evenly coat the bottom and sides of the pan.
5. Once the marshmallow mixture is ready, pour it into the prepared pan, using a spatula to spread the mixture evenly into the pan. Dust the top of the marshmallows with the sugar-cornstarch mixture to lightly cover, reserve the rest for later. Let the marshmallows sit uncovered for at least 4 hours or overnight.
6. After the marshmallows have set, use an offset spatula to carefully loosen the edges of the marshmallow from the pan. Turn the marshmallows out onto a cutting board. Cut the marshmallows into 1-2" squares using a sharp knife or pizza cutter lightly coated in the powdered sugar mixture. Dust all sides of the marshmallows with the powdered sugar mixture to prevent them from sticking together. Marshmallows can be kept in an airtight container at room temperature for up to 3 weeks.