Coat an 8x8 or 9x9-inch square baking pan with powdered sugar.
SOFTEN GELATIN:
In a large bowl, pour in ½ cup of cold water. Sprinkle the gelatin on top of the water. Set the bowl aside so the gelatin softens.
MAKE MARSHMALLOW BASE:
Add a candy thermometer to the side of a medium saucepan. Pour in ½ cup water, sugar, corn syrup, and salt.
Bring the mixture to a gentle boil over medium heat. Cook until the thermometer reads 240 degrees F (soft ball).
WHIP MARSHMALLOW:
Fit a stand mixer with the whisk attachment and turn the mixer on low. Slowly and carefully pour the hot sugar mixture into the mixer.
Once the entire sugar mixture has been added, turn the mixer to high until stiff peaks form, about 10 minutes.
Add vanilla to the thick marshmallow mixture.
ADD TO PAN:
Pour the marshmallow into the prepared baking pan and spread the mixture into an even layer. You’ll want to move quickly before the marshmallow starts to thicken. The marshmallows will sticky, so you’ll want to wet your fingers to help spread the mixture.
Sprinkle the top with additional powdered sugar.
LET MARSHMALLOW SET UP:
Let marshmallows set out overnight at room temperature.
CUT AND SERVE:
To cut, add 3-4 Tablespoons of powdered sugar to a large cookie sheet or cutting board.
Loosen the marshmallow from the edge of the pan using a sharp paring knife.
Invert the marshmallow pan onto the powdered sugar.
Sprinkle marshmallow with additional powdered sugar.
Cut the marshmallow into 1-inch squares and toss to coat with additional powdered sugar.
Shake excess powdered sugar off and store marshmallows in an airtight container at room temperature for 5-7 days.