Preheat the oven to 392℉ (200℃). Gather aluminum foil to wrap the sweet potatoes snugly for roasting.

Wrap each Japanese sweet potato in aluminum foil and roast on the oven rack for 1 hour and 10 minutes, until tender.

In a medium saucepan, combine ⅔ cup sugar, ¼ cup cornstarch, and ¼ teaspoon salt. Gradually whisk in 3 cups milk over medium heat until thickened.

In a small bowl, beat 3 egg yolks and slowly whisk in ¼ cup of the hot milk mixture. Return to saucepan and cook until thickened.

Remove custard from heat; whisk in 2 tablespoons unsalted butter and 1 teaspoon vanilla extract. Cover to cool.

Once sweet potatoes are roasted, cool for 10 minutes, slice off tops, and scoop out ⅓ of the flesh.

Fill the sweet potato shells with cooled custard mixture, smoothing the tops.

Sprinkle 1 teaspoon of caster sugar over each filled sweet potato. Caramelize using a kitchen blowtorch.

Allow the caramelized tops to cool slightly before serving.

Serve the Japanese Sweet Potato Crème Brûlée on a platter, optionally pairing with whipped cream or vanilla ice cream.