Place the rhubarb, water and sugar in a medium saucepan over medium heat. Stir periodically for 5-10 minutes until the rhubarb has broken down into a puree.
NOTE: taste the puree and if desired more sugar may be added.
Place a strainer or sieve over a bowl and pour rhubarb mixture into the strainer. With a spatula or pliable spoon, push the rhubarb through the strainer to release the juice.
Allow strained juice to cool. NOTE: pulp is not used in the recipe but may be used for other recipes (see blog post).
Instructions to make the Cocktail:
Pour 1/4 cup granulated sugar with a dusting of ground cinnamon in a small plate. Mix to combine.
Rim cocktail glasses with the sugar mixture.
Mix 1 ounce rhubarb puree with 2 ounces Vodka in a shaker with ice. Shake, strain over fresh ice in a lowball glass. Fill with club soda.