Soak the rice in plenty of water for at least 30 minutes. Drain well. Blend the rice with ⅔ cup water until completely smooth.

Transfer the mixture to a microwave-safe container. Microwave until it puffs up. Flip it, then microwave again until the back side also puffs up. Continue microwaving until no white liquid remains and the whole mixture turns an even off-white color.

Dust a cutting board generously with cornstarch. Transfer the hot mochi onto the board, flip it to coat both sides, and gently stretch it out.

Cut into pieces with a wet knife, re-wetting the blade between each cut.