Preheat your oven to 400°F (200°C). Wash and poke holes in the Japanese sweet potatoes with a fork, roast for 45–50 minutes until fork-tender, then mash until smooth.

Whisk the egg yolks with granulated sugar until pale and thickened. Warm the heavy cream in a saucepan until it starts to steam. Gradually add to the egg mixture, whisking continuously; stir in vanilla.

Fold the cooled, mashed sweet potatoes into the custard until no lumps remain. The mixture should be smooth and creamy.

Preheat the oven to 325°F (160°C). Prepare a water bath with your ramekins. Fill them with custard mixture about three-quarters full, then add hot water to the baking dish until halfway up the sides of the ramekins. Bake for 30–40 minutes until set with a slight jiggle.

Allow the ramekins to cool for 30 minutes at room temperature, then refrigerate for at least 2 hours. Before serving, sprinkle sugar on top and caramelize with a torch until golden brown.