Preheat your oven to 355°F (180°C). Place the Japanese sweet potatoes on a baking sheet lined with parchment paper and bake them for about 1 hour, or until they are tender.

Once the sweet potatoes are cool enough to handle, peel and mash them in a large mixing bowl until smooth. Incorporate egg yolks, cornstarch, sugar, and heavy whipping cream. Whisk until creamy, ensuring no lumps remain.

Transfer the mixture to a saucepan over medium heat. Stir continuously for about 5–7 minutes until slightly thickened and coats the back of the spatula.

Divide the custard into individual ramekins, cool on the countertop for 15 minutes, then refrigerate for at least 2 hours to fully set.

Remove the custards from the refrigerator, sprinkle with granulated sugar, and use a kitchen torch to caramelize the sugar until golden and crunchy.

Allow the caramelized sugar crust to cool for a minute. Serve immediately, optionally with fresh berries.