Mix glutinous rice flour, sugar, and matcha in a bowl. Gradually add water until smooth.
Cover bowl with plastic wrap, leaving a small gap for steam.
Microwave for 1 minute, stir, then microwave again for another minute until translucent.
Dust surface with cornstarch, transfer mochi, and flatten to ¼ inch.
Cut into squares or circles. Add 1 teaspoon of filling in center.
Pinch edges to seal, roll gently into a ball, and dust with cornstarch.