To make the syrup, put the rhubarb, sugar, and 1 1/4 cups water in a medium saucepan. Bring to a boil over medium heat, stirring occasionally until the sugar dissolves, reduce the heat to a simmer and cook until rhubarb is soft and falling apart, about 20-25 minutes.

Strain syrup through a fine mesh sieve and let syrup cool before using. You can either discard the rhubarb pulp or save it and use as a jam on toast or spooned over yogurt. Syrup will keep covered in the refrigerator for up to 3 weeks.

To make the drink combine all the ingredients in a cocktail shaker with ice. Shake vigorously until the outside of the shaker becomes frosty. Strain into a chilled glass and serve.