Grease and line a 26 x 17cm slice tray with baking paper. Extend the baking paper several centimetres above the pan, so you can lift it out once set.
Place the half cup of water in a small dish and sprinkle the gelatine over it. Whisk with a fork and set aside.
Place caster sugar and ¾ cup water in a saucepan. Cook over a low heat until sugar has dissolved. Increase heat and bring to the boil. Cook a further 8-10 mins, or until temperature reaches 118°C on sugar thermometer. Remove from heat and stir in the gelatine mixture, stirring until well combined.
Place egg whites into a large bowl of an electric mixer. Add salt and beat until soft peaks have formed. Gradually add the hot sugar mixture in a steady stream. Mixture will triple in size. Add vanilla and continue beating for 5 mins. Pour into the prepared pan. Allow to set at room temperature for at least 4 hrs before cutting.
Sieve the icing sugar and cornflour into a tray. Cut marshmallow with a wet sharp knife and toss in icing sugar mixture. Store in an airtight container.