For a less sweet cocktail, reduce the amount of simple syrup or omit it altogether. Taste and adjust to your preference. If you don't have fresh rosemary, a grapefruit slice or twist can also be used as a garnish. To make simple syrup, combine equal parts sugar and water in a saucepan. Heat over medium heat, stirring until the sugar dissolves completely. Let cool before using. For a festive touch, consider adding a splash of Prosecco or sparkling wine to each glass before serving. The cocktail can be made up to 24 hours in advance. Keep refrigerated until ready to serve.