This is the classic flan, which is as light and delicious as it is timeless. This basic flan recipe is also used to make flans of different flavors by substituting another liquid for part or all of the milk. For example, the best sweet oranges in Spain come from Valencia and Murcia, and an orange flan, made from freshly squeezed juice, is popular there.
Because flan takes a while to cool, it is usually prepared early in the morning or on the previous evening. When making it for a large group, it's practical to prepare a single large flan in a 5 by 9-inch mold and slice it for serving, rather than use individual cups. This recipe works well in the large mold and will bake in about the same amount of time.
For the flan:
For the caramelized sugar coating:
Preheat the oven to 300ºF.
To prepare the caramelized sugar coating, spread the sugar evenly in the bottom of a heavy saucepan and place over medium-low heat. It may take several minutes before the sugar begins to melt. Without stirring, watch the sugar closely as it begins to liquefy at the edges. All of it will slowly turn first into a yellowish and then golden syrup and finally into a brown caramel sauce.
When the liquefied sugar is turning from golden to brown, immediately remove the saucepan from the heat. (If you miss this point, the sugar will quickly turn too dark and taste bitter and you will need to discard it and begin again.)
Working swiftly, pour the liquid caramel into 12 custard cups each 3 1/2 inches in diameter and tilt to cover the bottom and sides evenly. It is important to do this transfer quickly, as the change in temperature causes the caramel to solidify rapidly. Set aside.
In a saucepan, combine the milk, lemon zest, and cinnamon stick over high heat and bring to a boil. Immediately decrease the heat to low and simmer for 10 minutes to infuse the milk with the flavor of the seasonings. Remove from the heat and let cool.
In a bowl, combine the whole eggs, egg yolks, and granulated sugar and whisk until foamy. Pour the cooled milk through a fine-mesh sieve held over the egg mixture and whisk until well blended. Pour the mixture into the coated custard cups.
Arrange the custard cups, not touching, in a large, deep baking pan or roasting pan. Pull out the oven rack, put the baking pan on it, and pour boiling water to a depth of about 1 inch into the pan to create a water bath. Bake for about 1 1/2 hours, or until set when tested with a thin-bladed knife in the center. Carefully remove the water bath from the oven, and then carefully remove the custards from the water bath and set aside to cool completely.
You can cover and refrigerate the cooled custards and serve them cold, or you can serve them at room temperature. One at a time, run the knife around the inside of each cup to loosen the edges of the custard and then invert the flan onto a dessert plate.