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1
Oil bottom and sides of a 13- by 9- by 2-inch rectangular metal baking pan and dust bottom and sides with some confectioners' sugar.
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2
In bowl of a standing electric mixer or in a large bowl sprinkle gelatin over cold water and let stand to soften.
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3
In a 3-quart heavy saucepan cook granulated sugar, corn syrup, hot water, and salt over low heat, stirring with a wooden spoon, until sugar is dissolved.
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4
Increase heat to moderate and boil mixture, without stirring, until a candy or digital thermometer registers 240F., about 12 minutes.
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5
Remove pan from heat and pour sugar mixture over gelatin mixture, stirring until gelatin is dissolved.
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6
With standing or a hand-held electric mixer beat mixture on high speed until white, thick, and nearly tripled in volume, about 6 minutes if using standing mixer or about 10 minutes if using hand-held mixer.
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7
In a large bowl with cleaned beaters beat whites (or reconstituted powdered whites) until they just hold stiff peaks.
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8
Beat whites and vanilla into sugar mixture until just combined.
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9
Pour mixture into baking pan and sift 1/4 cup confectioners― sugar evenly over top.
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10
Chill marshmallow, uncovered, until firm, at least 3 hours, and up to 1 day.
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11
Run a thin knife around edges of pan and invert pan onto a large cutting board.
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12
Lifting up 1 corner of inverted pan, with fingers loosen marshmallow and let drop onto cutting board.
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13
With a large knife trim edges of marshmallow and cut marshmallow into roughly 1-inch cubes.
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14
Sift remaining confectioners' sugar into a large bowl and add marshmallows in batches, tossing to evenly coat.
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15
Marshmallows keep in an airtight container at cool room temperature 1 week.
