Add the rhubarb, sugar, and water to a medium-size pot over medium to high heat, stirring occasionally. Once it reaches a boil, reduce the heat to a simmer, and add the vinegar, cinnamon and cardamom. Stir, then let the mixture simmer for at least 30 minutes, or up to an hour. The longer it simmers the more syrupy it will become.
Strain into a bottle or mason jar and let it cool to room temperature, then seal and refrigerate. This will keep in the fridge for weeks. But you’ll probably use it all before that.
Rhubarb Whiskey Sour
Add the whiskey, lemon juice, rhubarb syrup, and egg white to a cocktail shaker with a handful of ice. Shake well and strain into a chilled coupe glass. Garnish with a maraschino cherry (or don’t, because they’re gross, but so pretty!) and enjoy.