Add limoncello, vodka, lemon juice, and simple syrup to a cocktail shaker with ice.
Shake vigorously, for a full 30 seconds, to perfectly chill and dilute.
Strain into a stemmed cocktail glass, such as a martini glass or coupe glass. Garnish with a lemon slice or twist, and serve right away.
Notes
Feel free to use store-bought limoncello. For a special treat, make a batch of my homemade limoncello! Just know that every limoncello is going to have a slightly different flavor, so adjust the other ingredients accordingly (for example, with a sweeter limoncello I might not add any simple syrup at all).
Nutrition information is automatically calculated by Spoonacular. I am not a nutritionist and cannot guarantee accuracy. If your health depends on nutrition information, please calculate again with your own favorite calculator.