Toast cornstarch in a pan over medium-low heat for a few minutes to kill any bacteria (this is a safety precaution as you won’t be cooking this later). Spread evenly over a parchment paper-lined baking sheet. Sprinkle on the confectioners’ sugar. Reserve to coat cooked mochi.
Whisk all the mochi ingredients except the butter in a microwave-safe bowl until combined and not lumpy. Microwave on high power for 1 to 2 minutes. Mix again. Microwave for another minute and mix again. If there are any uncooked parts left, microwave for an additional minute. The mochi is ready when it is slightly translucent, cohesive, and evenly cooked. Add the butter and mix thoroughly.
Cover and cool until the mochi is no longer too hot to touch, about 8 minutes.
Wear food-safe gloves. Knead and stretch the mochi for about 15 seconds. It’ll be sticky.
Transfer the mochi to the prepared baking sheet. Coat lightly with the cornstarch and sugar mixture.
Roll the mochi into a log. Divide it into 8 even pieces. Working one at a time, flatten each piece into a disc. Tuck the disc onto itself by pinching in the dough from all sides into the middle. This creates a round shape. Place the mochi ball seam-side down on the work surface. With your fingers cupping the mochi, roll in a circular motion to firmly shape the ball. You can also roll it between your palms. Note the mochi may flatten a bit and not hold its shape well without any filling, and that’s okay. Perfection is overrated anyway.
Place on a serving plate. If desired, dust with matcha or cocoa powder. For added sweetness, drizzle with honey or condensed milk.
Store in an airtight container and eat within 2 days.