Whisk the egg whites in a large heat proof bowl using electric beaters. Whisk until soft peaks form then set aside. Put the gelatine in a deep bowl or jug and cover with 200ml cold water to soften.
Put the caster sugar, liquid glucose and 300ml water in a large, high-sided saucepan. Cook over a medium-high heat until the mixture reaches 130C on a sugar thermometer. Be very careful when you work with hot sugar. Take the pan off the heat then add the gelatine and the water they were soaked in to the hot sugar. Take care or wear oven gloves as the sugar can bubble up and spit. Stir until the gelatine has dissolved then carefully pour the mixture into a heatproof jug.
Return the beaters to egg whites and whip up further until stiff peaks form. Keep whisking while you slowly pour in the warm syrup in a steady stream. Keep beating the mixture until it is smooth and shiny, then add the vanilla seeds. Continue to use the electric beaters for around 8-10mins or until the mixture is noticeably thicker.
Line a 25cm x 35cm roasting tin (or any large and deep rectangular dish) with cling film and brush with sunflower oil. Mix the icing sugar and cornflour together then sieve a third of the mixture into the tray to coat the inside. Pour in the marshmallow mixture, level with a spatula and leave to set for 2 hours.
Spread a large sheet of baking parchment over your surface and sieve another third of the cornflour sugar mix over it. Upturn the set marshmallow onto the dusted sheet and peel away the cling film. Dust with a little more of the cornflour sugar and dust a large sharp knife with it too.
Cut the marshmallows into small squares approx. 3cm x 3cm sieving a little more cornflour sugar over all cut sides and knife as you go. You may not need all of it but they need to be coated on all sides otherwise they will stick. Serve straightway or keep in an airtight container for up to 2 days, separated with layers of baking parchment.