Preheat your oven to 355°F (180°C). Scrub the Japanese sweet potatoes clean and place them on a baking sheet. Bake for about 1 hour, or until the potatoes are tender when pierced with a fork.

Once the baked sweet potatoes are cool enough to handle, peel off the skin and place the flesh in a mixing bowl. Use a potato masher or fork to mash them until smooth.

In a large mixing bowl, combine the smooth sweet potato mash with the egg yolks, cornstarch, sugar, and heavy whipping cream. Whisk thoroughly until well blended.

Pour the mixture into a saucepan and cook it over medium heat, stirring constantly for about 5-7 minutes, until the custard thickens slightly.

Remove the custard from heat, let it cool at room temperature, then pour into individual ramekins and refrigerate for at least 2 hours.

After chilling, sprinkle an even layer of granulated sugar over each custard. Using a kitchen torch, caramelize the sugar until it melts and bubbles, ensuring an evenly browned texture.