Place the gelatine and 100ml/3½fl oz of cold water into a bowl. Set aside for 10 minutes, or until softened.
Meanwhile, pour the sugar and 175ml/6fl oz of cold water into a heavy-based saucepan and bring to a rolling boil on a medium to high heat, stirring continuously, until the sugar dissolves.
Reduce the heat to low and simmer the mixture gently for 15-20 minutes, until a sugar thermometer dipped into the mixture reads 113C/235F. (NB: At this temperature, the sugar syrup will have reached the soft-ball stage.) CAUTION: This mixture is extremely hot and can be dangerous. Do not leave unattended.
As soon as the sugar syrup reaches the correct temperature, place the softened gelatine and water into the bowl of a food mixer and blend on its lowest setting. Gradually pour in the sugar syrup in a slow, steady stream, avoiding pouring the sugar syrup on the beaters as it may splash. Whisk continuously until all of the syrup has been fully incorporated in the mixture.
Add the vanilla extract, then increase the blending speed and blend for 18-20 minutes, or until the mixture has thickened, cooled and is beginning to set.
Meanwhile, grease a 20cm/8in square, deep-sided cake tin with some of the sunflower oil. Line the tin baking parchment and grease the paper with the remaining oil. Mix together the icing sugar and cornflour and dust the tin with a little of the mixture to evenly coat the base and sides. Reserve the remaining icing sugar and cornflour mixture for later.
Pour the marshmallow mixture into the prepared tin and smooth the surface with a palette knife or spatula that has been dipped in boiling water. Dust the top of the marshmallows with a little more of the icing sugar and cornflour mixture, then cover the tin with cling film and set aside in a cool, dry place for 1-2 hours or overnight to set. (NB: Do not chill in the fridge.)
Once the marshmallow mixture has set, turn out the marshmallow slab onto a clean work surface dusted with the remaining icing sugar and cornflour mixture. Peel off the parchment paper and cut the marshmallow into 36 cubes. Dredge the marshmallows in the icing sugar and cornflour mixture to coat, or coat one or more sides in the chopped nuts, flower petals or coloured sugar for decoration. Store in an airtight container in a cool, dry place for up to three weeks.