Bring sugar and water to a boil in a small saucepan. Remove from heat, and add mint leaves. Let steep 10 minutes. Strain through a fine sieve into a small bowl, and discard mint. Let cool.
Using a vegetable peeler, remove zest from grapefruit in long, wide strips. Place in a large glass, and add rum. Cover, and let stand for 30 minutes. Strain through a fine sieve intoa large glass measure, and discard zest.
Cut away remaining pith from grapefruit. Cut between membranes to release segments. For each drink, place 2 to 3 segments in a glass. Top with 3/4 cup ice. Stir 3 tablespoons infused rum, 2 tablespoons mint syrup, and 1/4 cup seltzer in a glass measure, and pour over ice. Garnish with mint sprigs.
This was great. I followed the measurements perfectly and then tried to play with the proportions (ie more alcohol.) I think it works best if you stick to the recipe. I added a pinch of lemon to each glass. I used all of the excess grapefruit, not just the zest, to infuse the rum. I left it in the fridge over night and it was way better. I also left a few sprigs of mint in the simple syrup overnight.
I'm wondering how long you could leave the grapefruit in the rum. I'm thinking it would make a nice gift with the recipe attached. Thanks