Separate the leaves of gelatine and tip into a glass bowl. Pour over 150ml of boiling water from a kettle and stir until the gelatine has completely dissolved – if its not dissolving give it 10 sec blasts in the microwave and stir until it is. Scrape the mixture into the bowl of a table top mixer with a whisk attachment.
Tip the caster sugar and liquid glucose into a saucepan with 200ml water boil vigorously until the mixture reaches 127c on a cooking thermometer. When the syrup is up to temperature turn the whisk down to a slowish beat and slowly pour the hot syrup over the gelatine mixture. Add the vanilla extract and continue to whisk on high for 8 mins until the mixture has thickened to a point that it is shiny white and holds its shape.
Lightly grease a 20 x 20 cm tin. Use a grease spatula to scrape the mixture into the tin and use a piece of cling film to flatten it out. Leave for at least an hour to set. Loosen the edges with a knife and dust a work surface with the rest of the icing sugar mixture. Turn the marshmallow onto the icing sugar and use a knife dipped in boiling water to cut into squares – dust them with more of the icing sugar mix. Will keep in an airtight container for up to a week.