Prepare the Pan: Lightly grease a baking dish (9×9-inch works well) and dust with a mixture of 1/2 cup powdered sugar and 1/2 cup cornstarch. This will prevent the marshmallows from sticking to the pan once set.
Prepare the Gelatin: In a large mixing bowl, sprinkle 3 packets (21g) of unflavored gelatin over 1/2 cup of cold water. Let it sit for about 10 minutes to bloom, absorbing the water and softening.
Make the Syrup: In a saucepan, combine 2 cups of granulated sugar, 2/3 cup of light corn syrup (or golden syrup), 1/4 teaspoon salt, and 1/4 cup water. Bring the mixture to a boil over medium heat. Continue boiling until the syrup reaches 115°C (240°F) on a candy thermometer. This temperature is crucial for achieving the right texture.
Combine Gelatin and Syrup: Once the syrup reaches 115°C, carefully pour it into the gelatin mixture while mixing on low speed (using a hand mixer or stand mixer). Once the hot syrup is added, increase the speed to high and beat for about 10-12 minutes, until the mixture becomes thick, fluffy, and forms stiff peaks.
Add Vanilla: Add 1 tablespoon of pure vanilla extract to the marshmallow mixture and mix briefly to incorporate the flavor.
Set the Marshmallows: Pour the marshmallow mixture into the prepared pan, spreading it evenly with a spatula. Smooth the top and dust the surface with more of the powdered sugar and cornstarch mixture.
Let the Marshmallows Set: Let the marshmallows set uncovered for 6 hours or overnight at room temperature. This allows them to firm up and hold their shape.
Cut and Coat: Once set, cut the marshmallows into squares using a sharp knife or kitchen scissors. Coat each piece in the remaining sugar and cornstarch mixture to prevent them from sticking together.
