Instructions

1. 1
   Oil the bottom and sides of a rectangular metal baking pan, and dust the bottom and sides with some confectioners' sugar.
2. 2
   In bowl of standing electric mixer or in a large bowl, sprinkle gelatin over cold water and let stand to soften.
3. 3
   In a 3 quart (2.8 L) heavy saucepan, cook granulated sugar, corn syrup, hot water, and salt over low heat, stirring with a wooden spoon, until sugar is dissolved.
4. 4
   Increase heat to moderate and boil mixture, without stirring, until a candy or digital thermometer registers , about 12 minutes. Remove pan from heat and pour sugar mixture over gelatin mixture, stirring until gelatin is dissolved.
5. 5
   Using a standing or a hand-held electric mixer, whip mixture on high speed until white, thick and nearly tripled in volume (about 6–10 minutes).
6. 6
   In a large bowl with cleaned beaters, whip whites (or reconstituted powdered whites) until they just hold stiff peaks. Beat whites and vanilla into sugar mixture until just combined.
7. 7
   Pour mixture into baking pan and sift ¼ cup confectioners' sugar evenly over top. Cool marshmallow, uncovered, until firm (at least 3 hours and up to 1 day).
8. 8
   Run a thin knife around edges of pan and invert pan onto large cutting board. Lifting up 1 corner of inverted pan, with fingers loosen marshmallow and let drop onto cutting board.
9. 9
   With a large knife trim edges of marshmallow and cut marshmallows into roughly 1-inch cubes. Sift remaining confectioners' sugar into a large bowl and add marshmallows in batches, tossing to evenly coat.