Line a 23cm square baking dish with lightly greased foil or cling film. Grease another piece of foil or cling film to cover the top; set aside.
Place 120ml of water in the bowl of an electric mixer; sprinkle gelatine on top of water to soak.
While gelatine is soaking, combine 60ml water, syrup and sugar in a saucepan; bring to the boil over medium heat. Boil the mixture hard for 1 minute.
Pour the hot sugar mixture into the gelatine mixture and beat on high for 12 minutes with electric mixer, until the mixture is fluffy and forms stiff peaks. Add vanilla and peppermint extracts; beat just until blended.
Pour the marshmallow mixture into the prepared baking dish using a greased spatula to smooth the top. Cover the marshmallows with the reserved greased foil or cling film; press down lightly to seal the covering to the top of the marshmallows.
Allow the marshmallows to rest for 4 hours or overnight. Mix together cornflour and icing sugar in a shallow dish. Using oiled scissors or an oiled kitchen knife to cut the marshmallow into long strips, then into 2cm squares. Dredge the marshmallows lightly in the cornflour mixture and store in an airtight container.