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 1/2 cup cold cold water, and let soften.

In a 3-quart heavy saucepan cook granulated sugar, corn syrup, second 1/2 cup of cold 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 240°F, about 12 minutes. Remove pan from heat and pour sugar mixture over gelatin mixture, stirring until gelatin is dissolved. (I don't have a candy thermometer, and the 12 minute mark was just perfect)

With a stand- or a hand-held electric mixer beat mixture on high speed until white, thick, and nearly tripled in volume.

In separate medium bowl with cleaned beaters beat egg whites until they just hold stiff peaks. Beat whites and vanilla into sugar mixture until just combined. Pour mixture into baking pan and don’t fret if you don’t get it all out. Sift 1/4 cup confectioners sugar evenly over top. Chill marshmallow, uncovered, until firm, at least three hours, and up to one day.

Run a thin knife around edges of pan and turn pan over onto a large cutting board. Lifting up one corner of inverted pan, with fingers loosen marshmallow and ease onto cutting board. With a large knife trim edges of marshmallow and cut marshmallow into roughly one-inch cubes. Sift remaining confectioners’ sugar back into your now-empty baking pan, and roll the marshmallows through it, on all six sides, before shaking off the excess and packing them away or using right away.

Marshmallows keep in an airtight container at cool room temperature 1 week.