Make the syrup by combining 1 cup chopped rhubarb, 1 cup granulated sugar, and 1 cup water in a saucepan. Bring the mixture to a gentle simmer, stirring until the sugar dissolves, then simmer for 10 to 15 minutes until the rhubarb is soft and the liquid is pink and flavorful.

Strain the cooked mixture through a fine-mesh sieve into a clean container. Avoid pressing the rhubarb too firmly if you want a clearer syrup, then cool completely before using.

Chill a martini or coupe glass while the syrup cools. Add 2 ounces vodka, 1 ounce rhubarb syrup, 1/2 ounce fresh lemon juice, and plenty of ice to a cocktail shaker.

Shake vigorously for 10 to 15 seconds so the drink becomes thoroughly chilled and lightly diluted. Strain into the chilled glass and finish with a thin rhubarb ribbon, lemon twist, or small rhubarb stalk.