Add limoncello, gin, Lillet Blanc, and fresh lemon juice (plus simple syrup if using).
Shake hard for 10–12 seconds until well chilled.
Double strain through a Hawthorne and fine mesh strainer into a chilled coupe or martini glass.
Express a lemon twist over the drink and garnish.
Notes
For a slightly lower-ABV version, reduce gin to 0.75 oz and increase Lillet Blanc to 1 oz. Optional: add a couple drops of orange bitters for extra depth. Serve immediately while ice-cold.