3/4 cup granulated sugar
1/3 cup corn syrup
1/4 cup cold water
1/8 tsp salt
1/2 tsp vanilla
1/4 cup warm water
2 7-g envelopes unflavoured gelatin powder
1/4 cup icing sugar
1 tsp cornstarch
Stir granulated sugar with corn syrup, cold water, and salt in a small saucepan. Set over high. Bring to a boil, and let boil without stirring for 1 minute. Then stir in vanilla. While waiting for syrup to boil, pour warm water into the bowl of a stand mixer with whisk attachment. Sprinkle in gelatin. Let stand 2 minutes to soften. Line an 8-in square baking dish with overhanging plastic wrap. Lightly spray with cooking spray.
Set mixer on high, then pour hot syrup in a stream down the side of the mixer bowl, beating gelatin mixture until it is thick and sticky and white peaks form, about 4 minutes.
Scrape into prepared pan, smoothing top as best you can with an oiled spatula or a metal offset spatula. Let set at room temperature for at least 2 hours.
Whisk icing sugar with cornstarch in a small bowl. Sift one-third of mixture over top of marshmallow to dust the surface. Turn out onto a cutting board. Discard plastic wrap. Sift half of remaining sugar mixture overtop to dust the surface. Slice into cubes about 1 in. square. Roll each cut marshmallow in remaining sugar mixture until all sides are coated.