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 (180C) oven for about
40 minutes, until the top has set and is golden brown. Serve hot, chilled,
or at room temperature. Serves 4 to 6.