Step 1

Line a 9×13-inch pan with parchment paper, the parchment should be longer than the pan so you can use it to pull the marshmallows out. Spray it liberally with non-stick cooking spray and set aside.

Step 2

In a small bowl, whisk together 1/2 cup powdered sugar and all of the cornstarch and set aside.

Step 3

Pour ½ cup of the cold water into the bowl of an electric mixer fitted with the whisk attachment. Sprinkle the gelatin over the water and let bloom until firm, about 10 minutes.

Step 4

Meanwhile, combine the remaining water and the corn syrup in a medium saucepan. Pour the sugar into the center of the saucepan and add the salt. Place the pan over medium-high heat and bring to a boil until the sugar has dissolved completely and the syrup reaches 240 degrees F, about 8-10 minutes.

Step 5

Turn the mixer on low speed and carefully pour the hot syrup directly into the bloomed gelatin, avoiding the side of the bowl as much as possible. Gradually increase the speed to high and whip until the mixture is very thick, white, and glossy, about 10 to 12 minutes, scraping down the bowl as needed. Add the vanilla extract and mix until just incorporated, no more than a minute.

Step 6

Working quickly, because the marshmallow will set up and become difficult to spoon out, scrape the mixture into your prepared pan using a silicone spatula that has been sprayed with non-stick cooking spray. Smooth the top into an even layer quickly, as the marshmallow is setting fast. Sift half of the powdered sugar-cornstarch mix over the marshmallow slab. Cover with plastic wrap loosely and let sit overnight at room temperature.

Step 7

The next day, turn the marshmallow out onto a cutting board with parchment paper over the board and peel off the foil. Sift the rest of the powdered sugar-cornstarch mix over the slab once again.

Step 8

Using a pizza cutter or a sharp knife that has been sprayed with non-stick cooking spray, cut into 1-inch strips one way, and then across the other way for square marshmallows. You could also use round cutters or shaped cookie cutters rubbed with cooking spray to cut them out as well.

Step 9

Place the remaining 1/2 powdered sugar in a large ziploc bag. Working with 3 or 4 marshmallows at a time, toss the marshmallows in the bag with the powdered sugar mixture, then toss in a fine-mesh strainer to remove any excess powder. Marshmallows can be stored at room temperature in an airtight container or zip top bag for up to 2 weeks.