Refrigerate or freeze a martini or coupe glass for 5 minutes.
Place 1 ¾ ounces limoncello, 1 ½ ounces cold heavy cream, 1 ounce vodka, ¾ ounce lemon juice, and ½ ounce simple syrup if using in a cocktail shaker. Seal the shaker (without ice) and shake vigorously for 10 seconds. (This is referred to as a 'dry shake.' It's good for incorporating the dairy before adding ice to the shaker.)
Add enough ice to fill the shaker halfway. Seal the shaker again and shake vigorously until the outside of the shaker is frosty, about 15 seconds. Pour through a strainer into the chilled glass. Garnish with 1 lemon peel or finely grated lemon zest.
