Follow these straightforward steps to create your Japanese Sweet Potato Crème Brûlée:

1. Preheat the Oven: Start by preheating your oven to 325°F (160°C).
2. Prepare Sweet Potatoes: Wash the sweet potatoes and place them in a pot of water. Boil for about 20 minutes or until soft. Let cool, then peel and mash them into a smooth purée.
3. Make Custard Mixture: In a saucepan, combine heavy cream, milk, and granulated sugar. Heat over medium-low until it begins to steam, stirring occasionally. Do not boil.
4. Combine Ingredients: In a separate bowl, whisk together the egg yolks, vanilla extract, and salt. Gradually add the warm cream mixture to the yolks, whisking continuously. Fold in the sweet potato purée until smooth.
5. Bake: Pour the custard into ramekins and place them in a baking dish. Fill the dish with hot water until it reaches halfway up the sides of the ramekins. Bake for 30 minutes or until the edges are set. Remove and let cool, then refrigerate for at least 30 minutes.
6. Caramelize Topping: Before serving, sprinkle a thin layer of brown sugar on top of each custard. Use a kitchen torch to caramelize the sugar until it is golden and crisp.

By following these steps, you'll create a luxurious dessert that will amaze anyone who tries it.