Preheat oven to 392°F (200°C). Wrap sweet potatoes in foil and roast for 1 hour and 10 minutes until tender throughout. Let cool for 10 minutes.

While potatoes roast, stir sugar, cornstarch, and salt together in a saucepan. Whisk in milk gradually until smooth.

Place saucepan over medium heat and cook, stirring often, until thickened into a creamy base.

In a small bowl, lightly beat the egg yolks. Slowly whisk in about 1/4 cup of the hot milk mixture to temper. Pour this back into the saucepan and stir well.

Continue cooking the custard for 2 more minutes until thicker. Take off the heat and stir in butter and vanilla until smooth. Let cool with plastic wrap pressed directly on top to prevent skin from forming.

Slice the tops off each sweet potato and scoop out about one-third of the insides to create room for the custard.

Fill each potato shell with custard. Smooth the surface if needed.

Sprinkle 1 teaspoon of caster sugar over each filled sweet potato. Use a kitchen torch to melt and caramelize the sugar until golden and crisp.

Allow the sugar topping to harden slightly before serving.