Start by stirring together the powdered sugar and cornstarch in a small bowl.
⅓ cup powdered sugar, ⅓ cup cornstarch
Then grease your baking dish with butter, and dust a little bit of the sugar and cornstarch mix on top.
½ tablespoon butter
In the bowl of a stand mixer, use a spatula to gently stir together the gelatin and ½ cup water. Let sit for 10 minutes while you make the simple syrup.
3 packets unflavored gelatin, 1½ cups water
Make the simple syrup by stirring together the rest of the water, sugar, and salt in a pot over medium heat until the sugar has completely dissolved and comes to a light boil, simmer until it reaches 240°F (115°C), then remove from the heat and let cool slightly.
1½ cups water, 2½ cups granulated sugar, ¼ teaspoon salt
To the mixer with the gelatin and water mixture, add the whisk attachment. Turn on low speed, then slowly add in the simple syrup.
Once all of the simple syrup is in your stand mixer bowl, increase the speed to a medium to high speed and mix for about 15 minutes, or until the mixture gets thick, white and fluffy.
Before stopping the mixer, add in the vanilla extract, and let it continue whisking on high until the mixture turns white and fluffy again.
1 tablespoon vanilla extract
Pour the mixture into your greased baking dish and spread evenly.
Let sit at room temperature until firm, about 4 hours.
Once set, dust the top with a few shakes of the cornstarch and sugar mixture.
Slice the marshmallow while in the dish, dipping the knife in your sugar and cornstarch mixture after each cut.
To make 48 large marshmallows, make 5 long cuts and 7 cuts across, this will create 6 columns and 8 rows of large marshmallows.
For mini marshmallows, cut each large marshmallow into 8 mini marshmallows.
Dip the sticky sides of the marshmallows in the sugar and cornstarch mixture.
Notes
Store at room temperature for 2-3 days for best results. Storing longer will result in an undesirable texture.