Step 1

Grease the bottom and sides of a square-edge 9x13 rectangular pan with butter or flavorless cooking oil. Generously dust the sides and bottom of the pan with powdered sugar. Set aside. Pour half a cup of cold water into the bowl of a standing mixer. Sprinkle the gelatin over the water and let stand to soften.

Step 2

In a heavy saucepan, combine the sugar, syrup, 1/2 cup cold water, and the salt. Cook over low heat, stirring occasionally with a wooden spoon until the sugar is dissolved. Increase the heat to medium and boil the mixture until a candy thermometer registers 240 degrees, about 12 minutes. Remove from the heat and pour the sugar mixture over the gelatin, stirring until the gelatin has dissolved.

Step 3

Beat this mixture on high speed until thick, white and almost tripled in volume, at least 6 minutes. Grease a second mixing bowl and then transfer the sticky white mixture to the greased bowl. Clean the beaters and used bowl well and then whip two egg whites until they hold stiff peaks. Add the vanilla and then transfer the fluffy sugar mixture into the beaten egg whites.

Step 4

Beat the egg whites into the sugar mixture, just until combined. Scoop this mixture into the greased and powdered baking pan and spread it out to fill the pan. Sift 1/4 cup powdered sugar evenly over the top of the marshmallows. Chill at least 3 hours or up to 24 hours.

Step 5

The brown rice syrup version will be ready to cut between 3-4 hours. The all sugar version requires at least 12 hours in the refrigerator - I typically leave them to chill overnight.

Step 6

Run a knife around the edges of the pan and then invert the pan over a large cutting board. Lift up one corner of the pan and use your fingers to release the marshmallow from the pan and place it on the cutting board. Dust the top again with powdered sugar if it is very sticky. With a pizza cutter, or a large knife, trim the edges of the marshmallows and then cut the marshmallows into approximately 1" cubes.

Step 7

Sift the remaining powdered sugar into the now-empty baking pan. Roll the marshmallows through it, on all six sides, before placing them in an airtight container. They will keep well at cool room temperature for at least 1 week.