Grease and line an 18cm (7inch) square, 2.5cm (1inch) deep cake tin with baking parchment. In a large, heatproof bowl, make up the Egg White Powder according to the packet instructions and then whisk until very stiff. Set aside.
Cut up the Gelatine Leaves into a small heatproof bowl. Add 60ml (4 tbsp) cold water. Leave to soak for 5 minutes then put the bowl over a saucepan of gently simmering water until the leaves have melted. Remove the bowl from the saucepan of water and set aside.
Put the sugar in a small saucepan with the Liquid Glucose and 50ml (2 fl.oz) water. Put a sugar thermometer in the saucepan. Heat the sugar gently, stirring until melted. Raise the heat slightly and let the mixture bubble until it becomes syrupy and reaches 118°C on the thermometer.
Remove the saucepan from the heat. Whisk the Egg White mixture again while gently pouring over the hot sugar syrup in a steady stream. Continue whisking as you gradually pour in the liquid Gelatine and add the Madagascan Vanilla Extract to form a thick, glossy meringue-like mixture (this takes about 5 minutes).
Scrape the fluffy mixture into the prepared tin using a spatula and smooth off the top. Leave in a cool, dry place (not the fridge) for 3 to 4 hours until firm and set.
To finish, dust the work surface with the cornflour, and turn the marshmallow on to it. Carefully peel away the parchment. Using a large bladed knife, cut into 5 strips and then 25 chunks (the mixture will be quite sticky). Toss the pieces of marshmallow in the cornflour until lightly coated and serve