Let's cook.
Peel the Japanese sweet potato and cut it into 1-inch cubes. Steam the sweet potato cubes until very tender, about 15-20 minutes. You should be able to easily mash them with a fork.
Transfer the steamed sweet potato to a large bowl and mash thoroughly until smooth. There should be no lumps.
Add the glutinous rice flour, granulated sugar, and salt to the mashed sweet potato. Mix well with a spatula until a dough forms. If the dough is too dry, add water one tablespoon at a time until it comes together.
Divide the dough into 12 equal pieces. Roll each piece into a small ball, then gently flatten it into a disc about 1/2-inch thick.
Heat the vegetable oil in a non-stick skillet over medium heat. Place the mochi discs in the skillet, ensuring they are not overcrowded. Cook for 3-4 minutes per side, or until golden brown and slightly puffed.
Remove the mochi from the skillet and place them on a plate lined with paper towels to absorb any excess oil. Serve warm.
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