1 Reduce the rhubarb juice by half and leave to cool, then soak the gelatine in the juice.
2 Place the juice and gelatine over a medium heat until the gelatine melts.
3 Boil the sugar, whisky, glucose, water and salt in a separate pot for 1½ minutes. Pour this boiling syrup onto the gelatine mixture.
4 Add the vanilla seeds and mix together, then whisk in an electric mixer for 12 minutes.
5 Pour into a tray lined with clingfilm, then refrigerate for 4 hours.
6 Cut into squares and dust with caster sugar.
• Perceptions by Mark Greenaway is out on 12 July, £30, see www.markgreenaway.co.uk