In the bottom of a glass, add half of the mint leaves and a small amount of the Torani Syrup. Muddle the mint leaves until they are crushed and fragrant. Then add the rum, rest of the Torani Syrup, lime juice and ice. Top off with the soda water and stir. Add more Torani if you'd like it a little sweeter. Garnish with grapefruit slices and additional mint leaves if you have extra.