Poach rhubarb in water with sugar until soft. Strain the liquid off into a jug and keep some of the rhubarb for serving.
Warm the strained rhubarb liquid and stir in the softened gelatine sheets or
powdered gelatine until dissolved. Stir in the vodka and Cointreau then strain
into 4 glasses. Chill until set. Decorate with poached rhubarb.