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1
Sift the powdered sugar and cornstarch or potato starch together into a large bowl; set aside.
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2
Lightly coat an 8-by-8-inch baking pan with cooking spray; set aside.
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3
Whisk together the gelatin and 1/2 cup of the cold water in a small microwave-safe bowl and let soften at room temperature for 5 minutes.
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4
Stir together the sugar, 1/4 cup of the corn syrup, remaining 1/4 cup water, and salt in a medium saucepan over high heat.
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5
Bring to a boil, stirring occasionally, until the temperature reaches 240 degrees F on a candy/fat thermometer.
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6
Meanwhile, pour the remaining 1/4 cup of the corn syrup into the bowl of a stand mixer fitted with a whisk attachment.
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7
Microwave the gelatin on high until completely melted, about 30 seconds.
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8
Pour it into the mixer bowl.
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9
Turn the mixer to low and keep it running.
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10
When the syrup reaches 240 degrees F, slowly pour it into the mixer bowl (the mixer should still be running).
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11
Increase the speed to medium and whip for 5 minutes.
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12
Increase the speed to medium high and whip 5 minutes more.
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13
Add the vanilla, increase the speed to the highest setting, and whip until opaque white, fluffy, and tripled in volume, about 1 to 2 minutes more.
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14
Pour the marshmallow mixture into the prepared pan and use an offset spatula to smooth it into the corners.
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15
Sift 1/2 cup of the coating evenly and generously over top; set the remaining coating aside.
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16
Let the marshmallow set uncovered for at least 6 hours in a cool, dry place.
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17
Use a knife to loosen the marshmallow slab from the edges of the pan.
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18
Dust a work surface with a thin layer of the reserved coating.
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19
Invert the marshmallow slab onto the work surface and dust it with more coating.
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20
Cut into whatever size pieces you wish (a pizza cutter works great for squares).
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21
Dip the sticky edges of the marshmallows in the remaining coating, patting off the excess.
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22
Store in an airtight container at room temperature for up to 1 month.
