Line the bottom and the sides of a 9-inch square baking pan with plastic wrap.
Lightly coat the plastic wrap with nonstick spray.
Using a fine mesh sieve, generously dust the prepared pan with the powdered sugar.
In the bowl of a stand mixer fitted with the whisk attachment, add the gelatin and 1/2 cup of the cold water and whisk to combine.
Allow the gelatin mixture to stand until it "blooms" or thickens, about 10 minutes.
In a small saucepan over medium-high heat, add the granulated sugar, the corn syrup, the kosher salt, and the remaining cold water and cook, while stirring often, until the sugar is dissolved, about 3 minutes.
Cook the sugar mixture, while stirring occasionally, until it thickens into a syrup and registers 240 degrees F on a candy thermometer, about 10 minutes.
Transfer the sugar mixture from the heat.
Immediately turn the stand mixer to medium-low speed and carefully drizzle the hot sugar mixture into the gelatin mixture while the mixer is running.
Once all of the sugar mixture is combined, gradually increase the mixing speed to high and beat until the mixture is very thick and fluffy, about 10-12 minutes.
Add the vanilla to the marshmallow mixture and stir to combine.
Pour the marshmallow mixture into the prepared pan and quickly smooth out the top.
Heavily dust the top of the marshmallow mixture with the powdered sugar.
Let the marshmallow mixture stand, uncovered, at room temperature for at least 8 hours and up to overnight.
In a medium bowl, add the powdered sugar.
Using the plastic wrap, lift the marshmallow block from the pan and transfer it to a cutting board.
Unwrap and discard the plastic wrap.
Using a large knife dusted with the powdered sugar to prevent sticking, cut the marshmallow block into 20 even squares.
In batches, toss the cut marshmallows in the powdered sugar in the bowl until they are no longer sticky.
Dust off the excess powdered sugar from the marshmallows and repeat until all the marshmallows are cut and coated.
Serve.