Butter the bottom & sides of a 9x13 pan and dust with powdered sugar

In the bowl of a standing mixer sprinkle the gelatin over cold water and let stand to soften

Meanwhile, in a heavy 3-quart sauce pan cook sugar, corn syrup, 1/2 cup water and salt over low heat

Stir with a wooden spoon until sugar is dissolved

Increase heat to moderate and boil mixture, without stirring, until a candy thermometer reaches 240 degrees F.

Remove from heat

Pour sugar mixture over gelatin mixture, stirring with a wooden spoon until gelatin is dissolved

With a standing mixer, beat mixture on high until white, thick and nearly tripled in volume.

In a separate bowl beat egg whites just until stiff peaks form.

Beat egg whites and vanilla into white fluffy mixture until just combined.

Pour into butter pan.

Sift 1/4 powdered sugar over top.

Chill, uncovered for at least 3 hours

Run a knife around the edge of pan

Lift up a corner with fingers and pull the marshmallow from the pan

On a cutting board, with a greased serrated knife cut into 1 inch cubes

roll the marshmallows in powdered sugar

Keep in an air tight container