Pour the sugar and 175ml (6fl oz) cold water into a heavy-based saucepan, place on a medium–high heat and bring to a rolling boil, stirring all the time until the sugar dissolves. Reduce the heat to low and allow to simmer for about 15–20 minutes without stirring, until a sugar thermometer dipped into the mixture reads 113°C. Alternatively, check that the soft-ball stage has been reached.
Pour the gelatine and 100ml cold water into the bowl of an electric food mixer and leave for about 10 minutes to soften before stirring to loosen up. As soon as the sugar syrup reaches the correct temperature, begin to whisk the gelatine in the food mixer on a very low setting and then carefully pour in the syrup in a slow, steady stream, whisking all until it is fully incorporated. Add the vanilla extract, increase the whisking speed and leave to beat for about 20 minutes until thick, cool and beginning to set.
Meanwhile, grease a 20cm square cake tin with some of the sunflower oil, line with parchment paper and grease again. Toss the icing sugar and cornflour together and place in a sieve. Dust a little into the tin to evenly coat the base and sides, reserving the rest for later.
Pour the creamy marshmallow mixture into the prepared tin, spreading it evenly and smoothing the surface using a palette knife dipped in boiling water. Dust with a little more of the icing sugar and cornflour mixture, cover with cling film and allow to set in a cool, dry place (not the fridge) for 1–2 hours or overnight.
Once the marshmallow mixture has set, dust a clean work surface with the remaining icing sugar and cornflour mixture and turn the marshmallow slab out of the tin. Peel off the parchment paper and cut the marshmallows into about 36 squares. Toss them around to coat evenly in the icing sugar and cornflour. Store in an airtight container in a cool, dry place for up to three weeks