Preheat your oven to 325°F (165°C) and gather your ingredients and tools: a mixing bowl, whisk, ramekins, and a deep baking dish.

In a mixing bowl, combine the mashed Japanese sweet potatoes, heavy cream, granulated sugar, egg yolks, and vanilla extract. Whisk vigorously for about 3–4 minutes until smooth and creamy.

Pour the custard mixture evenly into your prepared ramekins and place the ramekins into a deep baking dish. Fill the baking dish with hot water until it reaches halfway up the sides of the ramekins.

Bake for about 45 minutes until the custard is set but slightly wobbly in the center.

Remove the ramekins from the water bath and let cool at room temperature for about 30 minutes. Then cover the ramekins with plastic wrap and refrigerate for at least 2 hours.

Sprinkle a thin layer of granulated sugar over each custard and caramelize with a kitchen torch until golden brown.