Instead of vanilla, you may infuse the cream with 1 ½ -inch piece vanilla bean, split lengthwise. Scrape the vanilla seeds into the milk, add the bean pod, and heat the milk to a simmer. Cover, remove from the heat, and allow to infuse for 15 minutes. Remove he vanilla bean pod, heat to a simmer again, and proceed as directed.
Preheat the oven to 325°F.
Whisk 6 large egg yolks and 1/3 cup sugar in a medium bowl just until blended.
Bring 2 cups whole milk or half-and-half just to a simmer in a small saucepan.
When milk starts to steam, remove from heat and whisk in 5 ounces bittersweet chocolate (up to 64% cacao), finely chopped.
Gradually whisk the warm chocolate mixture into the egg yolk mixture. Strain through a fine-mesh sieve into a bowl or large measuring cup with a pouring lip. Skim off any foam with a spoon. Stir in 1 teaspoon vanilla.
Pour mixture into six 4-ounce ramekins. Cover each ramekin tightly with a piece of foil to prevent a skin from forming. Set the custards in a deep pan large enough to accommodate them without touching each other or the sides of the pan. Set the pan on a pulled-out oven rack and add enough scalding-hot tap water to come one-half to two-thirds of the way up the sides of the reamkins. Bake until set but still quivery in the center when shaken, 40–50 minutes.
Remove the custards from the water bath and let cool on a wire rack for 30 minutes, then refrigerate, covered, for at least 2 hours before serving. You may store them in the refrigerator for up to 2 days.