Prepare martini glass by lightly rubbing the rim with the fleshy wet part of a cut lemon, then dip the moistened rim of the glass into the sugar poured on a plate.
Fill ⅔ of a cocktail shaker with ice.
Pour the limoncello, vodka and freshly squeezed lemon juice into the shaker.
Shake vigorously until combined then pour into a chilled martini glass. Garnish with a slice of lemon.