Mix gelatin and cold water in a small bowl and set aside to bloom. In a medium-sized pot, begin warming skim milk, ginger, lemon grass and vanilla. As it's heating, quickly whisk in the bloomed gelatin when milk starts to bubble around the edges, remove pot from heat and set aside. Do not boil. In a large mixing bowl, whisk together agave nectar, buttermilk, yogurt and lime juice. Once the milk mixture has cooled slightly, add it to the mixing bowl, whisking well to combine. Strain through a fine mesh sieve and divide mixture into 4 oz. ramekins or martini glasses. Top with lime zest to taste. Place ramekins or glasses in the refrigerator to chill for at least 3-4 hours. Overnight is best.