Heat the oven to 400°F. Pierce the sweet potato and roast it whole for 45 to 60 minutes, until completely soft.

Peel the sweet potato and push the flesh through a fine-mesh sieve. Measure out ½ cup of puree.

Reduce the oven to 325°F. Heat the cream with the cinnamon and salt over medium heat until hot at the edges but not simmering.

Whisk the egg yolks and sugar for about 1 minute, until pale. Pour the hot cream into the yolks in a thin stream while whisking constantly.

Whisk in the sweet potato puree and vanilla, then strain the base through a fine-mesh sieve into a pitcher.

Divide between four 6-ounce ramekins set in a deep baking dish. Pour hot water into the dish until it comes halfway up the sides of the ramekins.

Bake for 35 to 45 minutes, until the edges are set and the centers wobble as one soft piece. Lift the ramekins out, cool 30 minutes, then refrigerate at least 4 hours or overnight.

Blot the custard surfaces dry with a paper towel. Sprinkle 1 to 1 ½ teaspoons of granulated sugar over each, tilting to spread it thin and even, and tip out any excess.

Torch in small circles, keeping the flame moving, until the sugar melts and turns deep amber. Rest 2 minutes so the crust hardens, then serve.