Bring the lime juice, vanilla seeds, sugar and water to the boil. Heat the butter in a pan. Add the rice, sauté while stirring until the rice is translucent. Gradually add the sugar & lime syrup, stirring frequently, so that the rice is always just covered with liquid, simmer for about 25 mins. until the risotto is creamy and al dente. Stir the lemon balm and butter into the risotto.