1 cup confectioners' sugar
3 1/2 envelopes (2 tbls. plus 2 1/2 teas.) unflavored gelatin
1/2 cup cold water
2 cups granulated sugar
1/2 cup light corn syrup
1/2 cup hot water (about 115F.)
1/4 teaspoon salt
2 large egg whites
1 teaspoon vanilla

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.
In bowl of a standing electric mixer or in a large bowl sprinkle
gelatin over cold water and let stand to soften.
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. Increase heat to moderate and boil mixture,
without stirring, until a candy or digital thermometer registers 240F,
about 12 minutes. Remove pan from heat and pour sugar mixture over
gelatin mixture, stirring until gelatin is dissolved.
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. In
a large bowl with cleaned beaters beat whites (or reconstituted powdered
whites) until they just hold stiff peaks. Beat whites and vanilla into
sugar mixture until just combined. Pour mixture into baking pan and
sift 1/4 cup confectioners sugar evenly over top. Chill marshmallow,
uncovered, until firm, at least 3 hours, and up to 1 day.
Run a thin knife around edges of pan and invert pan onto a large cutting
board. Lifting up 1 corner of inverted pan, with fingers loosen
marshmallow and let drop onto cutting board. With a large knife trim
edges of marshmallow and cut marshmallow into roughly 1−inch cubes.
Sift remaining confectioners' sugar into a large bowl and add
marshmallows in batches, tossing to evenly coat. Marshmallows​
keep in an airtight container at cool room temperature 1 week.