In a microwave-safe bowl, combine the glutinous rice flour, granulated sugar, and water. Whisk until smooth and no lumps remain.

Cover the bowl loosely with microwave-safe plastic wrap and poke a few small holes to allow steam to escape.

Microwave for 1 minute. Remove the bowl and stir thoroughly.

Microwave for another minute, then remove and stir again.

Add the butter and mix until fully incorporated into the mochi dough.

Microwave for an additional 30 seconds. Stir and repeat in 30-second intervals until the dough becomes thick, sticky, and slightly translucent.

Remove from the microwave and allow the mochi dough to cool until it is warm enough to handle.

Lightly dust a clean work surface with cornstarch to prevent sticking.

While the mochi dough is still slightly warm, transfer it to the prepared surface.

Gently knead the dough until smooth and stretchy.

Using a rolling pin, roll the mochi into a thin, even rectangle, approximately ⅛ inch thick.

Cut the dough into evenly sized squares.

Add the semisweet chocolate chips and optional butter to a microwave-safe bowl.

Microwave in 15–20-second intervals, stirring thoroughly between each interval.

Continue until the chocolate is completely melted and smooth. Avoid overheating, as chocolate can burn or seize.

Place one square of mochi onto a parchment-lined plate.

Spoon a small amount of melted chocolate onto the mochi and spread it into a thin, even layer.

Place another mochi square on top of the chocolate.

Continue alternating layers of mochi and melted chocolate until you reach your desired number of layers.

Finish with a layer of chocolate on top, if desired.

Transfer the assembled chocolate mochi to the freezer for approximately 15–20 minutes, or until the chocolate has set.

Remove from the freezer and allow it to sit at room temperature for a few minutes if the mochi has become too firm.