Grease and line an 18cm (7inch) square deep cake tin with baking parchment. Whisk the egg whites until very stiff. Set aside.
Cut up the Gelatine Leaves into a small heatproof bowl. Add 60ml cold water. Leave to soak for 5 minutes. Put the bowl over a saucepan of gently simmering water until the leaves have melted. Remove the bowl and set aside.
Put the sugar in a small saucepan with the Liquid Glucose and 50ml water. Put a sugar thermometer in the saucepan. Heat gently, stirring until the sugar has melted. Raise the heat slightly and let the mixture bubble until it becomes syrupy and reaches 118°C (245°F) on the thermometer. Remove from 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 Dr Oetker 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. Decorate with a combination of chocolate, sprinkles and chopped nuts.