Combine ice, limoncello, vodka, simple syrup, and lemon juice in a cocktail shaker. Close lid and shake until well combined and chilled.
Strain into a martini glass and garnish with a lemon peel.
Notes
This amount of simple syrup makes for a cocktail that’s on the sweeter side. Feel free to use more or less simple syrup to make your perfect martini.
To make simple syrup, simmer one cup sugar in one cup of water, stirring until the sugar is fully dissolved. Remove from the heat and chill until needed.