In the bottom of a tall glass add 6 mint leaves and a little bit of simple syrup. Muddle the mint leaves until they are crushed (and smell amazing!) Next, add rum, 1.5 ounces grapefruit simple syrup (you can add more at the end if it's not sweet enough for your taste), juice of one lime, ice and top off with club soda. Stir and taste. Add more syrup if you like a sweeter cocktail and stir again. Garnish with grapefruit slices and fresh mint leaves.
To make the grapefruit simple syrup, simply combine 1 cup grapefruit juice and 1 cup sugar in a small pan. Stir on medium heat until the sugar is completely dissolved. Refrigerate in an airtight container. Keeps for two weeks or more.