Preheat the oven to 325°F (163°C). Prepare a baking dish for a water bath.

Boil peeled and cubed Japanese sweet potatoes for about 15 minutes or until fork-tender. Drain and mash until smooth.

In a saucepan over low heat, combine heavy cream, whole milk, and vanilla extract. Warm for about 5 minutes without boiling.

Whisk 4 egg yolks in a separate bowl until pale and slightly frothy. Gradually add the warm cream mixture to the yolks, stirring constantly.

Fold the mashed sweet potatoes into the custard mixture until smooth.

Pour the custard into ramekins, filling them three-quarters full. Place in a baking dish and pour hot water around the ramekins to halfway up their sides.

Bake for 30-40 minutes until the edges are set but the center has a slight jiggle. Test for doneness with a knife.

Let the ramekins cool to room temperature, then cover and chill in the refrigerator for at least 2 hours.

Before serving, sprinkle sugar on top of each chilled crème brûlée and caramelize with a kitchen torch until golden and crisp.