How to Make It
In a small bowl, mix the gelatin with 1/3 cup of water and let stand for 15 minutes. Meanwhile, grease an 8-inch square cake pan with oil and dust lightly with cornstarch, tapping out the excess.
In a small saucepan, mix the sugar with 1/3 cup of water and bring to a boil, stirring to dissolve the sugar. Add the corn syrup and cook over moderately high heat, without stirring, until the syrup registers 260° on a candy thermometer. Off the heat, stir in the gelatin mixture until dissolved.
In a stand mixer fitted with the whisk, beat the egg whites at moderately high speed until soft peaks form. With the machine on, drizzle the hot syrup into the egg whites in a very thin stream down the side of the bowl. Scrape the marshmallow into the prepared pan, smoothing the surface. Let stand until set, at least 3 hours.
Run a sharp knife around the marshmallow. Very lightly dust a work surface with cornstarch and invert the cake pan onto it, tapping to release the marshmallow. Using a lightly greased sharp knife, cut the marshmallow into 1 1/2-inch pieces.
Cinnamon marshmallows: Add 1/2 teaspoon ground Vietnamese cinnamon during the last minute of beating the marshmallow; lightly dust the cut marshmallows with a sifted mixture of 2/3 cup cornstarch, 1/3 cup confectioners' sugar and 1 teaspoon ground cinnamon. Hazelnut marshmallows: Add 1/4 cup hazelnut praline paste during the last minute of beating the marshmallow; coat the cut marshmallows in coarsely ground roasted hazelnuts. Coconut-lime marshmallows: Add 1 teaspoon pure coconut extract and 2 teaspoons finely grated lime zest during the last minute of beating the marshmallow; coat the cut marshmallows with toasted unsweetened shredded coconut.