How to Make It
Place 1/2 cup of the water in a microwave-safe bowl; stir in gelatin. Set aside.
Combine sugar, corn syrup, salt, and remaining 3/4 cup water in a large saucepan over medium-high; bring to a boil, and cook, stirring occasionally, until sugar dissolves, about 5 minutes. Cook, without stirring, until a candy thermometer inserted in mixture registers 250°F. Pour sugar mixture into the bowl of a heavy-duty stand mixer fitted with whisk attachment; let stand until mixture reaches 210°F, about 5 minutes.
Microwave gelatin mixture on HIGH until gelatin melts, about 20 seconds, stirring after 10 seconds. Beating on low speed, gradually pour gelatin mixture in a thin stream into sugar mixture; add vanilla. Increase speed to high, and beat until light and fluffy, 9 to 10 minutes. Using a lightly greased spatula, spread mixture evenly in a lightly greased 13- x 9-inch glass or ceramic baking dish. Let stand until dry and firm, about 2 hours.
Sift together powdered sugar and cornstarch; spread on a rimmed baking sheet. Using a lightly greased offset spatula, remove marshmallow mixture from dish, and place in powdered sugar mixture. Using kitchen shears well coated with powdered sugar mixture, cut marshmallows into 1-inch squares. Coat all cut sides of marshmallows with powdered sugar mixture, tossing lightly to remove excess. Store marshmallows in an airtight container.