1. Lightly butter a 9-inch (23cm) loaf pan. Have a pan or baking dish with high sides ready, that’s larger than the loaf pan, for making a bain-marie (water bath) to bake the custard in. A roasting pan or large baking dish works well.
2. To make the caramel, spread the 1 cup (200g) of sugar in an even layer in a skillet. Pour the water over it and add a few drops of lemon juice. Cook the sugar over medium-high heat, swirling the pan only as necessary, to let the water moisten the sugar, and so it cooks evenly. (Avoid stirring or over-swirling; excessive movement can cause the sugar to crystallize.)
3. When the sugar is liquefied, and starts to turn a deep amber color and begins to smoke, remove it from the heat and pour the caramel into the buttered mold. Tilt the mold so the caramel coats the bottom evenly, being careful with the very hot caramel. Set aside to let cool.
4. Preheat the oven to 300ºF (150ºC).
5. Whisk the eggs and 2/3 cup (130g) of sugar in a large bowl. Scrape the seeds from the vanilla bean and add them to the eggs. (The vanilla pod can be reserved for another use.) Gradually add the milk to the eggs, whisking constantly as you pour.
6. When the custard mixture is well-blended, pour it into the prepared loaf pan. Place the custard-filled pan inside the larger roasting pan or baking dish, then pour very warm water into the outer pan or dish until it reaches a little more than halfway up the outside of the loaf pan. Bake the custard in the oven 1 hour, 20 minutes, uncovered, until the center feels like it’s just barely set.
7. Remove the custard from the oven, and let it rest in the water bath for 30 minutes. Transfer the pan with the custard in it to a cooling rack, and let cool to room temperature, then refrigerate until completely chilled, preferably overnight.