Start with a shallow dish of sugar. Go around the rim of your chilled glass with a slice of lemon to wet it then dip the rim of the glass into the sugar making sure you have an even coat of sugar all the way around.
Add your vodka, limoncello, lemon simple syrup and a handful of ice to a cocktail shaker. Shake vigorously for 30 seconds or until very cold. Strain into the prepared glass and serve immediately.