Heat the oven to 375°F (190°C), arrange the potatoes on a parchment-lined large baking pan, pierce their tops with a fork, and roast for 45–55 minutes before cooling enough to handle. A fork should enter the flesh easily, with no hard center.

Gently press each warm potato from above to flatten its base, remove a slice of skin from the top, and spoon out the interior while retaining a ½-inch wall. The hollowed potato should sit level and hold its bowl shape.

Mash or blend ½ cup of the removed flesh, using additional mashed flesh to press closed any accidental holes in the shells. The measured puree should have no firm lumps, and each patch should close the opening.

Use a large pitcher or measuring cup to stir the yolks, custard sugar, and measured puree into a uniform mixture. Look for an even color without streaks of yolk or pockets of potato.

Stir in the salt, cinnamon, vanilla, whipping cream, and milk without beating air into the mixture. The filling should pour smoothly rather than carry a thick cap of foam.

Distribute the filling among the potato bowls, reserving any excess for a ramekin baked by the water-bath method below. Check that the shells remain level and that no filling leaks onto the pan.

Reduce the oven to 340°F (170°C), bake for 20–25 minutes with adjustments for potato size, then cool completely and refrigerate for 2–4 hours or overnight until set. At the end of baking, the perimeter should hold still while the center trembles like soft gelatin rather than sloshing like liquid.

Roast one sweet potato at 375°F (190°C) for 45–55 minutes, then let it cool and mash or blend the flesh for the measured puree. The cooked interior should yield readily to a fork and mash without hard pieces.

Stir the egg yolks and custard sugar together in a large pitcher or measuring cup. A smooth, evenly colored mixture is the goal, not a pale, whipped foam.

Incorporate the salt, cinnamon, vanilla, measured sweet potato puree, whipping cream, and milk with gentle stirring. No visible ribbons of puree or cream should remain.

Divide the custard among 4–6 oven-safe ramekins, nestle them in a tall-sided baking pan, and carefully pour boiling water around them without splashing the filling. The water should surround the vessels while staying outside the custard.

Bake at 340°F (170°C) for 18–22 minutes, then cool completely and refrigerate for 2–4 hours or overnight until set. Remove the custards while their edges look stable and their centers still move with a gentle, cohesive wobble.

Scatter 1–2 teaspoons of granulated sugar over each chilled custard and spread it across the exposed surface. A level coating melts more evenly than scattered piles or thick patches.

Immediately before serving, sweep a kitchen torch across the sugar, keeping the flame moving and about an inch clear of dry potato-skin edges, or use the broiler for broiler-rated ramekins while watching continuously. Once the topping becomes a glossy amber sheet and hardens, serve promptly for a crisp crack.