Cook the Sweet Potato: Peel the sweet potato and cut it into small pieces. Soak them in water for about 5 minutes, then drain. Steam or boil until tender.
Make the Dough: Transfer the cooked sweet potato to a bowl and mash with a potato masher or fork until smooth. Add the soy milk, potato starch, and vegetable oil, then mix until a smooth dough forms.
Shape the Balls: Take a small portion of the dough and roll it between your palms to form a small ball. Repeat with the remaining dough.
Deep-Fry: Heat the frying oil to 170°C (340°F). Fry the sweet potato balls in batches for about 4 minutes, or until golden brown and crispy. Remove and drain well.
Coat with Sugar: Roll the donut holes in sugar while they are still warm. Serve and enjoy!
Notes
If the mixture is dry and crumbly, add soy milk 1 teaspoon at a time until the dough is smooth and easy to shape. If it is too soft or sticky, add a little more potato starch.
Mash the sweet potato while it is still warm. This makes it easier to create a smooth dough without lumps.
Roll the dough firmly but gently between your palms, smoothing out any large cracks. This helps prevent the donut holes from breaking apart while frying.