Dust a large sheet tray or a 33cmx22cm (13inx9in) baking dish with an equal amount of icing sugar and corn starch. Or grease liberally with cooking spray or butter. Set aside.
Add 30ml (2 tablespoons) of cool water to a small bowl. Dust the top of the water with the gelatin, ensuring it is evenly covered. Allow to sit and hydrate for at least 10 minutes.
In a small saucepan combine the sugar along with 60ml (¼ cup) of cool water. Set over medium-low to medium heat. Stirring constantly, slowly dissolve the sugar in the water, ensuring that the sugar is completely dissolved before it comes to a boil.
Once the sugar syrup comes to a boil, stop stirring and wash down any errant crystals collected on the sides of the pan with a pastry brush. Cook the syrup until it reaches 115°C (240°F).
While the syrup is coming to temperature, whip your egg whites to soft peaks in a stand mixer fitted with the whisk attachment (or in a large bowl using a hand mixer - see note 2).
As soon as the sugar syrup has come to 115°C (240°F), carefully stream it into the eggwhite with the mixer running at medium speed. Once all of the syrup is incorporated, turn the mixer to medium-high and whip the meringue until it is glossy, has reached stiff peaks, and the exterior of the missing bowl feels cool to the touch.
While the meringue is finishing whipping, add the softened gelatin to the saucepan that held the sugar syrup (there is no need to wash it) and gently melt the gelatin over very low heat until clear and liquid.
Once the meringue has reached stiff peaks, add the melted gelatin and vanilla and whip until just combined, about 1 minute longer.
Transfer the marshmallows to a pastry bag and pipe individual marshmallows onto the prepared baking sheet. Or, scrape the entirety of the marshmallow from the bowl into the prepared 33cmx22cm baking dish and even out the top with a spatula. Dust with more icing sugar, if necessary.
Allow the marshmallows to set for at least 4 hours before cutting into squares and serving.
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Notes
I recommend using caster sugar (also known as superfine sugar) here as it dissolves more easily and quickly in the water. However, you can use granulated sugar if this is all you have.
If you're using a hand mixer rather than a stand mixer, it can be helpful to have a friend stream the syrup into the bowl while someone else holds the mixer, rather than trying to do it all by yourself!