1 cup ice cubes, or as needed
1 1/2 fluid ounces hazelnut liqueur
1 1/2 fluid ounces vanilla-flavored vodka
1 fluid ounce simple syrup
1 tablespoon half-and-half, or to taste
Fill a cocktail shaker with ice. Pour hazelnut liqueur, vodka, simple syrup, and half-and-half over ice. Cover shaker and shake until liquid is cold; strain into a martini glass.