Measure all the ingredients. Dust a clean work surface and tray with cornstarch or potato starch to prevent sticking.
In a microwave-safe bowl, whisk together the glutinous rice flour, sugar, water, and vegetable oil until the batter is completely smooth and free of lumps.
Cover the bowl loosely with plastic wrap or a microwave-safe lid. Microwave for 1 minute, stir, then continue microwaving in 1-minute intervals, stirring after each, until the dough becomes thick, glossy, stretchy, and fully cooked (about 3–4 minutes total).
Transfer the hot dough onto the prepared work surface. Let it cool slightly, dust the top with more starch, then roll or flatten it to about ¼ inch (6 mm) thick.
Use a round cutter or knife to divide the dough into equal portions. Gently shape each piece into smooth, round mochi, dusting lightly with starch as needed to prevent sticking.
Brush off any excess starch and arrange the mochi on a serving plate. Serve plain or enjoy with tea.
