You can’t go wrong this holiday when you finish on a sweet note with this traditional Mexican flan.
Caramelizing the molds: Choose 6 6-ounce or 4 8-ounce molds—custard cups, coffee cups or individual soufflé dishes. Set them into a baking pan at least 2 inches deep and large enough to give the molds at least ½ inch clearance all around.
Into a small (1- to 1 ½-quart) saucepan, measure the ¾ cup of sugar. Dribble 1/3 cup water around and over the sugar, evenly moistening it, then set on medium-high heat. When the mixture comes to a full boil, wash down the sides of the pan with a brush dipped in water (this dissolves any clinging sugar crystals). Reduce the heat to medium and boil without stirring until the syrup begins to turn golden, 3 to 5 minutes. Now, carefully start gently swirling the pan over the heat until the syrup is a deep straw color. Remove from the heat and continue swirling until the color is a rich amber. Quickly pour a portion of caramel into each one of the molds. Immediately tilt the molds to evenly cover the bottom with caramel. (To clean the saucepan, fill it with water and set over medium heat to dissolve the stuck-on caramel.)
The custard mixture: Heat the oven to 325º, position the oven rack in the middle, bring a kettle of water to a simmer and choose which style of flan you are going to make. In a medium-size (2 ½- to 3-quart) saucepan, combine the sugar called for in the flan version of your choice, the milk and the half-and-half or condensed milk (whichever you’re using). Set over medium heat, add the lime zest if you’re using it, and stir as the mixture comes to a simmer. Remove from the heat. If you’re using lime zest, cover and let steep 10 minutes.
In a large bowl, whisk the eggs (or egg and yolk combination) until liquidy, then slowly whisk in the warm milk mixture. Stir in the vanilla and strain through a fine-mesh strainer into a pitcher or bowl. Pour or ladle a portion of the custard mixture into each of the molds.
Baking the custards. Pull out the oven rack and set the pan holding the filled molds onto it. Carefully pour the simmering water into the baking pan, letting it fill around the molds to a depth of about 2 inches. Carefully push the rack into the oven, close the door and bake until the custards are barely set in the middle, 50 to 60 minutes for small molds, 60 to 70 minutes for larger ones. I recommend turning the pan around about half way through baking.
Let the custards cool in their hot water baths, which will take about an hour, so they slowly set completely.
Serving the flans: Refrigerate them for at least 2 hours before serving: To serve, run a small knife around the top edge of each flan, penetrating about ½ inch below the surface. Quickly turn each mold over onto a serving plate. One by one, grasp plate and mold firmly and shake up-and-down, back-and-forth, until you hear the flan drop onto the plate. Remove the mold and scrape out sticky dissolving caramel from the inside, letting it drizzle down on the flan. (Putting the empty molds to soak in very hot water will help remove any undissolved caramel.)
Working Ahead: Covered and refrigerated, flans will keep very well for 4 or 5 days.