Rinse the Mochigome under cold water until the rinse water runs clear, then soak the grains in enough cold Water to cover for 6–8 hours or overnight so they absorb moisture evenly.
Drain the soaked rice and transfer to a steamer basket; steam over boiling water for about 40–60 minutes (or until the grains are soft and translucent) with a lid on to trap the steam.
While the rice is still hot, transfer it to a large mortar (usu) or heatproof bowl. Begin pounding or vigorously mashing the steamed rice with a heavy pestle or wooden mallet; the grains will first crush and then come together into a sticky mass.
As you work the rice, wet your hands occasionally with a little Water to keep the dough from drying and to help it become smooth and elastic—add only drops at a time until you reach a cohesive, glossy consistency.
Turn the mochi out onto a surface lightly dusted with Potato Starch to prevent sticking; use additional starch to dust your hands and the work surface liberally.
Divide the mochi into portions with a wet knife or scissors, shaping each piece into rounds or desired shapes by gently stretching and tucking the dough under itself.
If you plan to fill them, flatten each piece, add a small spoonful of filling, then pinch closed and roll lightly in more Potato Starch to finish. Serve fresh at room temperature.
