How to Make It
Line a mini muffin pan with plastic wrap. Scoop twelve 2-tablespoon portions of green tea ice cream into each cup. Freeze until very firm, about 2 hours.
Using a damp cloth, wipe down a large work surface. Stretch a 14-inch sheet of plastic wrap over the surface and use the damp cloth to smooth it out. Sift a generous layer of cornstarch over the plastic wrap.
In a medium microwave-safe bowl, whisk the rice flour with the sugar, matcha and salt. Whisk in the water in a slow steady stream and continue whisking until a smooth paste forms. Cover the bowl tightly with plastic wrap. Microwave at high for 2 minutes. Uncover and stir the mixture until it is mostly smooth. Tightly cover the bowl again with the plastic wrap and microwave at high for 2 minutes longer.
Scrape the mochi onto the prepared surface and dust with more cornstarch. Roll the mochi out 1/8 inch thick and let cool slightly. Using a 3-inch round cutter, stamp out 12 rounds of mochi, rerolling the scraps. Using a pastry brush, dust off the excess cornstarch.
Working with one round at a time, lightly brush the edge with water. Place the round in the palm of your hand. Take 1 scoop of ice cream out of the freezer and place in the center of the mochi. Wrap the mochi around the ice cream and twist to seal. Transfer the mocha ice cream ball back to the muffin pan seam side down and return to the freezer. Repeat with the remaining rounds and ice cream. Freeze until firm, about 1 hour.