Prepare the Sweet Potatoes: Start by wrapping the Japanese sweet potatoes in baking paper and aluminum foil. Bake them in an oven preheated to 355°F (180°C) for about an hour until they become soft and tender.

Scoop and Mash: Once baked, carefully remove the sweet potatoes from the oven. Cut an oval shape from the top of each potato and scoop out some of the flesh. Mash the scooped sweet potato with a fork until smooth and creamy.

Combine Ingredients: In a mixing bowl, whisk together the egg yolks, sugar, and cornstarch until smooth. Gradually stir in the heavy cream, mixing well to combine all the ingredients into a silky custard.

Cook the Custard: Transfer the mixture to a small pot and cook over medium heat. Stir continuously until the custard thickens, and you see small bubbles forming. This should take about 8-10 minutes.

Fill the Potatoes: Carefully fill each sweet potato skin with the creamy custard you just made, allowing them to cool slightly before proceeding.

(Optional) Oven Finish: If you're aiming for that classic crème brûlée touch, pop the filled sweet potatoes back into the oven for an additional 30 minutes. Alternatively, refrigerate them for a refreshing cold treat.

Caramelize the Sugar: Finally, sprinkle a thin layer of sugar on top of the custard in each potato. Using a kitchen torch, caramelize the sugar until it forms a beautiful, golden crust.