For an extra layer of flavor, infuse your heavy cream with vanilla bean overnight before whipping. This intensifies the vanilla notes. Adjust the amount of ice to achieve your preferred thickness. For a thinner consistency, use less ice; for a thicker, almost milkshake-like texture, use more. If you don't have lemon and elderflower vodka, plain vodka with a dash of elderflower liqueur works well as a substitute. To make this ahead of time, blend all ingredients except the ice. Store the mixture in the refrigerator, and then blend with ice just before serving.