Wash and peel the Japanese sweet potatoes. Cut into even pieces and steam or boil in salted water for 15-20 minutes until tender.

In a saucepan, combine heavy cream, granulated sugar, and vanilla extract. Heat over medium-low for about 5 minutes.

Whisk the egg yolks until smooth, then temper by adding a ladle of the warm cream mixture.

Pour the warmed egg mixture back into the saucepan with the cream and fold in the sweet potato mash.

Preheat oven to 325°F (160°C). Distribute custard mixture into ramekins and pour water into baking dish.

Bake for 30-40 minutes until edges are set but centers jiggle slightly.

Cool them at room temperature, then chill in the refrigerator for at least 4 hours.

Sprinkle granulated sugar on top and caramelize using a kitchen torch or under the broiler.