Put oven rack in middle position and preheat oven to 350°F.

Cook sugar in a dry small heavy saucepan over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork, until sugar melts into a deep golden caramel. Immediately pour into a 9-inch round ceramic or glass baking dish or metal cake pan (2 inches deep) and tilt dish to coat bottom (use caution, dish will be hot). Cool until hardened, 10 to 15 minutes.

Blend remaining ingredients in a blender, in 2 batches if your blender is small, until smooth. Pour custard through a fine-mesh sieve over caramel in dish, then transfer dish to a 17- by 11-inch roasting pan lined with a kitchen towel. Cover dish loosely with a piece of foil, then pour enough boiling-hot water into roasting pan to reach 1 inch up side of dish. Bake until custard is set but still wobbly in center when gently shaken and a knife inserted in center comes out clean, 1 to 1 1/4 hours. Transfer dish to a rack to cool completely, about 40 minutes. Chill flan, covered, until cold, at least 8 hours.

To unmold flan, run a thin knife around edge of dish to loosen flan. Invert a large platter with a lip over dish. Holding dish and platter securely together, quickly invert and turn out flan onto platter. Caramel will pour out over and around flan.

Cooks' note:
Flan can be chilled up to 1 day (before unmolding).

Makes 6 to 8 servings.