Let's cook.
Steam the peeled and diced sweet potatoes until very tender, about 15-20 minutes. You should be able to easily mash them with a fork.
Transfer the steamed sweet potatoes to a large bowl. Add the granulated sugar and salt, then mash thoroughly until smooth. Let it cool slightly.
Once cooled, add the glutinous rice flour, all-purpose flour, and milk to the sweet potato mixture. Mix and knead until a soft, pliable dough forms. If the dough is too sticky, add a tiny bit more all-purpose flour; if too dry, add a few drops more milk.
Divide the dough into 8 equal portions. Roll each portion into a smooth ball, then gently flatten it into a disc about 1/2 inch thick.
Heat the vegetable oil in a deep pot or Dutch oven over medium heat until it reaches 350°F (175°C).
Carefully place 3-4 donuts into the hot oil, ensuring not to overcrowd the pot. Fry for 3-4 minutes, flipping occasionally, until golden brown and puffed. Adjust heat as needed to maintain oil temperature.
Remove the fried donuts with a slotted spoon and place them on a wire rack lined with paper towels to drain excess oil. Serve warm.
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