Boil the diced Japanese sweet potatoes in water for 10-15 minutes until tender, then mash until smooth.
Heat heavy cream and milk in a saucepan over medium heat until it simmers.
Whisk egg yolks and vanilla extract in a mixing bowl until smooth. Gradually add warm cream and milk, whisking continuously.
Fold the mashed sweet potatoes into the egg mixture until fully combined, then pour into ramekins.
Set the ramekins in a baking dish and fill with hot water halfway up the sides.
Bake for 40-45 minutes, until edges are firm and centers jiggle slightly.
Chill the ramekins at room temperature for 30 minutes, then refrigerate for at least 2 hours.
Sprinkle granulated sugar on top of each custard and caramelize with a kitchen torch until golden-brown.
Allow caramelized sugar to cool before serving.
Notes
For best results, use fresh, firm Japanese sweet potatoes. Remember to gently fold the sweet potatoes into the egg mixture to keep the custard light and creamy.