Add base spirit, rhubarb syrup, lemon juice, and egg white to a cocktail shaker. Dry shake for 15-20 seconds.
Add ice to the shaker and shake again. Strain into a coupe glass. Garnish with rhubarb ribbon (created by running a vegetable peeler down the length of a stalk of rhubarb).
In saucepan over medium-high heat, bring rhubarb, sugar, 1 cup water and lemon peel to boil, stirring occasionally. Reduce heat to medium-low and simmer, stirring occasionally, until rhubarb breaks down and mixture thickens slightly, about 5-8 minutes.
Strain into bowl, pressing solids gently to extract liquid; discard solids. Cover and refrigerate until chilled.Make-ahead: Refrigerate in airtight container for up to 5 days.
Rhubarb syrup recipe adapted from Canadian Living, June 11, 2008.