Preheat the oven to 300°F (150°C) if using gratin dishes or 325ºF (160ºC) if using ramekins or custard cups.
Place 4 individual gratin dishes or six 4-ounce ramekins or custard cups on a high-rimmed baking sheet (also known as a jellyroll pan) or in a roasting pan large enough to contain all of them. In a small saucepan over medium heat, warm the cream, milk, sugar, and salt until the sugar dissolves, which shouldn't take long at all.
In a bowl, whisk together the egg yolks.
Gradually add the warm cream mixture to the egg yolks in a slow, steady stream, gently (not vigorously) whisking until the cream is completely incorporated but without creating any foam. Mix in the espresso or coffee powder and strain the mixture into a large measuring cup or another vessel with a spout. Stir in the liqueur. Divide the mixture among the gratin dishes, ramekins, or custard cups.
If using gratin dishes, place the pan containing the custards on the oven rack and pour enough hot water into the pan so that it reaches at least halfway up the sides of the gratin dishes. Bake the custards for 20 to 25 minutes, or until they are just set; watch them very carefully during the final few minutes of baking.
If using ramekins or custard cups, the dishes will not be filled completely. Place the pan containing the custards on the oven rack and pour enough hot water into the pan so that it reaches at least halfway up the sides of the ramekins or cups. Snugly cover the baking pan with aluminum foil and bake the custards for 30 to 35 minutes. When you jiggle the pan, they should just barely quiver.
Remove the dishes from the pan and set them on a cooling rack. (A wide metal spatula works well for lifting the hot custards from the water; be careful, as the custards are hot.) Let the custards cool to room temperature. Then loosely cover and refrigerate overnight.
Just before serving, sprinkle the top of each crème brûlée with an even layer of sugar. It should be enough to cover the top, but not too heavily—1 1/2 teaspoons for each crème brûlée is about right. Using a blowtorch (the kind that you kind from a hardware store is perfect), wave the flame over each custard, 1 at a time, until the sugar melts and then browns. You may need to lift and tilt the dishes so that the caramel flows evenly across the top. If so, be extremely careful because the caramel is very hot and any drips will cause a painful burn. Serve immediately. Originally published October 17, 2014.