Bring sugar and water to a boil in a small heavy saucepan over medium heat, stirring until sugar has dissolved, then wash down any sugar crystals from side of pan with a pastry brush dipped in cold water. Boil, without stirring, swirling pan occasionally so caramel colors evenly, until dark amber.
Immediately pour into ramekins and swirl so caramel coats bottom of each, then put ramekins in a large roasting pan.
Preheat oven to 350°F with rack in middle.
Blend 2 cups milk with remaining flan ingredients in a blender until smooth, about 1 minute.
Transfer to a bowl and stir in remaining 1 1/2 cups milk, then divide among ramekins.
Bake flan in a water bath until just set but still wobbly in center, 1 to 1 1/4 hours.
Remove ramekins from water bath and cool slightly on a rack, at least 15 minutes (flan will continue to set as it cools).
Just before serving, run a thin knife around each flan, then invert onto plates.
Flan can be made 2 days ahead. Chill in ramekins, uncovered, until cool, then covered.
Let stand at room temperature about 1 hour before serving.
