Preparation

• Position a rack in the center of the oven and heat the oven to 325°F. Put 6 shallow 4-oz. crème brûlée dishes or ramekins in one large or two small roasting pans. Bring a kettle of water to a boil. In a 3-quart saucepan, bring the cream to a bare simmer over medium heat. Remove from the heat and set aside.
• In a large mixing bowl, whisk the egg yolks, sugar, vanilla, and salt until combined. Whisking slowly so as not to over-aerate, add 1/2 cup of the warm cream to the egg mixture. Slowly whisk in the rest of the cream, then the sweet potatoes, until combined.
• Divide the mixture among the ramekins. Pull out the oven rack and place the roasting pan on the  rack. Carefully pour the hot water from the kettle into the pan to come halfway up the sides of the ramekins. Bake until the custards are set and firm around the edges with a little jiggle in the center, 30 to 40 minutes.
• With tongs or a spatula, carefully transfer the ramekins to a rack to cool completely. Cover and refrigerate for at least 4 hours and up to 2 days.
• Just before serving, sprinkle one of the custards evenly with 2 to 3 tsp. sugar. (The more sugar, the thicker and darker the crust will be.) Pass a kitchen torch over the surface until the sugar melts and caramelizes. Repeat with the remaining custards. Allow the sugar to cool and harden briefly, then serve.