Pour ½ cup of the water into a small bowl.Sprinkle the gelatin powder into the water; stir, then set aside.
In a large pot, combine the granulated sugar,corn syrup, salt, and the remaining ½ cup of water. Bring to a boil over mediumhigh heat. Reduce the heat to medium and simmer, stirring often, until thecandy reaches 250 degrees Fahrenheit on a candy thermometer.
Very carefully, pour the hot candy mixture intoa large mixing bowl. Safety note: be extremely cautious to not splash the veryhot candy onto your skin!
Add the gelatin and water mixture to the candy. Beatthe candy with an electric mixer (a stand mixer is preferable if possible),starting on low speed and gradually increasing the speed to high as the mixturethickens. When the marshmallow has thickened and turned white, add the vanillaextract. Continue to beat the marshmallow on high speed until it has more thandoubled in volume.
Prepare an 11x7 inch glass dish by completelycovering the bottom and sides with powdered sugar. Use your fingers to pressthe powdered sugar up the sides of the pan. You’ll want every bit of the pan tobe coated in powdered sugar so that the marshmallow does not stick.
Scoop the marshmallow into the pan prepared withthe powdered sugar. Use a lightly oiled spoon to smooth the marshmallow out,then sprinkle the top of the marshmallow generously with powdered sugar.
Let set up in a cool dry place for 8 hours, Cut the marshmallow lengthwise into 5 longstrips, then cut each strip into 10 pieces to create a total of 50 marshmallowsquares. Roll each marshmallow in powdered sugar.
Store marshmallows at room temperature in anairtight container.