Sift together the cornstarch and icing sugar into a bowl and set aside for later.
In a small cup, sprinkle the gelatin over 125 mL of water. Set aside.
In a small saucepan, place the 80 mL of water, followed by the granulated sugar and corn syrup. Attach your candy thermometer to the saucepan, making sure that the tip of the probe is not touching the bottom of the pan. Begin heating the syrup over medium–high heat.
Place the egg whites with a pinch of salt in the bowl of an electric mixer, fitted with the whisk attachment. Beat them on low til frothy.
When the syrup boils, increase the mixer speed to high until the whites form stiff peaks.
When the syrup reaches 245°F, take the saucepan off the heat and remove the thermometer, and then slowly pour the syrup down the side of the mixer bowl and over the whites, with the mixer running on low, being sure to not hit the beaters with the syrup.
When the syrup is all added, increase the mixer speed to medium–high. Meanwhile, add the bloomed gelatin mixture to the saucepan and melt it over very low heat til the solution is clear, then add the dissolved gelatin to the mixer bowl, along with the vanilla.
Beat the marshmallow mixture until it is cold.
Dust a parchment-lined rimmed quarter sheet pan with half the icing sugar mixture prepared earlier, then pour the marshmallow mixture on top and smooth it.
Let the marshmallows dry for several hours before cutting them (with well oiled scissors or oiled cookie cutters). Immediately toss the marshmallows in the powdered sugar mixture before storing.