Mix sweet rice flour and green tea powder thoroughly in a microwave-safe bowl. Stir in water, then sugar. Mix until smooth. Cover bowl with plastic wrap.
Cook in the microwave for 3 minutes. Then remove the plastic wrap. The consistency will be almost rubbery.
Dust work surface with cornstarch. While the mochi is still hot from the microwave, begin rolling balls the size of about 2 tablespoons. Flatten the mochi ball and place white chocolate in the center.
Pinch the mochi over chocolate to seal. Sprinkle with a dusting of sifted powdered sugar and place mochi seam side down in a paper muffin liner to prevent sticking. Let cool before serving.