Two teaspoons of vanilla extract, whisked into the yolks in step 4, can be substituted for the vanilla bean. The best way to judge doneness is with a digital instant-read thermometer. The custards, especially if baked in shallow fluted dishes, will not be deep enough to provide an accurate reading with a dial-face thermometer. For the caramelized sugar crust, we recommend turbinado or Demerara sugar. Regular granulated sugar will work, too, but use only 1 scant teaspoon on each ramekin or 1 1/2 teaspoons on each shallow fluted dish. If your oven has a history of uneven heating, the custards may finish at different rates, so it is advisable to check each one separately rather than take the whole lot out at once.