Stir together the flour, sugar and salt in a small bowl and set aside. In a large bowl combine the lime juice, milk, egg yolks, and lime zest, stirring to combine thoroughly. Add the dry ingredients to the wet ingredients and stir to form a smooth, liquid batter. Stir in the melted butter. Beat the egg whites until stiff but not dry. Gently fold the beaten egg whites into the batter until thoroughly combined. Pour the batter into a buttered baking dish. Place this baking dish inside a larger baking dish, and add hot water to the larger dish until it reaches one inch (2.5 cm) up the side of the smaller baking dish. Bake in a pre-heated 350F (180 °C) oven for about 40 minutes, until the top has set and is golden brown. Serve hot, chilled, or at room temperature.