Grease a 9” by 13” non-stick pan with oil spray. Put 1/2 cup water in your stand mixer bowl. Sprinkle in gelatin and let mixture sit until needed.
Place remaining 1⁄2 cup water, granulated sugar, syrup, vanilla and salt in a heavy pot. Set over high heat, bring to rapid boil, boil 2 minutes, and then remove from heat.
Using the whip attachment and low speed, slowly mix the hot sugar mixture into the gelatin mixture. Turn speed to medium-high and whip 3 minutes. Now turn speed to high. Whip 7 minutes more, or until mixture at least triples in size and is very thick, but still warm.
With a spatula, spread marshmallow into prepared pan. Grease a sheet of plastic wrap, larger than the pan, with oil spray. Set it, greased-side-down, on the marshmallow. Press on the plastic wrap, sealing the marshmallow underneath. Let marshmallow set at room temperature at least eight hours.
To cut, sprinkle a work surface 1/2 cup icing sugar. Use a paring knife to loosen the marshmallow from the pan. Now quickly invert onto the work surface. Rub the top and sides of marshmallow with 1/4 cup icing sugar. Put remaining icing sugar in a shallow bowl.
Line a baking sheet with parchment paper. Cut marshmallow into 48, 1 1/2” squares. Coat each cut marshmallow with icing sugar, setting them on the baking sheet as you go along.
Let marshmallows sit uncovered two hours to dry and set exteriors. Pack marshmallows, not stacked, in airtight containers at room temperature until needed.
Note: Envelopes of Knox brand gelatin were used in this recipe. It is sold in small boxes in our baking supply aisle.