In a microwave-safe bowl, whisk together the sweet rice flour, matcha powder, sugar, and salt until no lumps remain. Pour in the water and whisk until completely smooth with no dry pockets.
Cover the bowl loosely with plastic wrap and microwave on high for 1 minute. Remove and stir vigorously with a wet spatula. The mixture will be partially cooked and slightly thickened.
Microwave again for 1 more minute on high, then stir thoroughly. The dough should look glossy, translucent, and stretchy at this point with no white chalky spots.
Prepare your work surface by dusting it generously with cornstarch. Turn the hot mochi dough out onto the cornstarch and dust the top heavily as well.
Using a cornstarch-dusted rolling pin or your hands, gently flatten the mochi into a rectangle about 1/2-inch thick. Work quickly while it's still warm and pliable.
Cut into 8-10 equal squares or circles using a sharp knife or cookie cutter dusted with cornstarch. Lift each piece carefully and dust all sides with more cornstarch to prevent sticking.
Let the mochi cool completely at room temperature for 10-15 minutes before serving. The texture will firm up slightly as it cools but remain soft and chewy.
