Grease a 3cm-deep, 16.5cm x 26cm (base) slab pan. Line base and sides with baking paper, allowing a 2cm overhang at both long ends.
Combine sugar and ⅔ cup hot water in a saucepan over medium heat.
Cook, stirring, for 3 minutes or until sugar dissolves and syrup is clear.
Using a fork, combine gelatin and ⅔ cup cold water in a jug. Pour into hot syrup.
Cook, stirring, for 2 to 3 minutes or until gelatin has dissolved and mixture is clear. Pour into bowl. Set aside to cool to room temperature (about 30 minutes).
Using an electric mixer, beat sugar and gelatin mixture on high for 6 to 10 minutes or until very thick. (the mixture should become white in color and nearly triples in volume)
Add vanilla and food coloring (to your desired color) to the mixture, beat for 1 minute.
Spread into prepared pan. Smooth top and sprinkle with powdered sugar. Set aside, at room temperature, for 1 hour or until set.
Using a wet knife, cut into squares pieces of marshmallow .
Serving Size 12 Servings