To make the ganache, add the chocolate, butter, and heavy cream to a microwave-safe bowl and put it in a microwave set to 600 watts for one minute and thirty seconds.
Let the mixture rest for an additional thirty seconds, and then stir together until it's smooth and free of lumps. Cover and refrigerate until solid (about 4 hours).
To make the mochi, line a 5x5-inch pan with parchment paper.
In a large microwave-safe bowl, whisk together the mochi flour, cocoa powder, and sugar until evenly combined.
Whisk in the milk a little at a time until the mixture is smooth and free of lumps. Add the remaining milk and whisk until smooth.
Put the mochi mixture into the microwave and set it at 600 watts for two minutes and thirty seconds.
Use the whisk to break up any solidified lumps and add the butter. Whisk together until the butter is melted and emulsified with the mixture.
Microwave the chocolate mochi at 600 watts for an additional three minutes and thirty seconds.
Use a spatula or wet wooden paddle to knead the mochi until it is smooth and elastic. It is extremely hot and sticky, so be careful not to get any on your hands.
Dump the chocolate mochi into the prepared mold, spread the mixture evenly into the corners, and flatten off the top. Let the mochi cool to room temperature, and then cover while you're waiting for the ganache to set.
When the ganache has firmed up, use a knife to cut the chocolate mochi into quarters in both directions to make 16 squares that are roughly the same size.
Use a round 1/2 tablespoon or melon baller to scoop a ball of ganache out of the bowl.
Use your thumbs and fingers to press out a piece of mochi into a 2-inch disk that's roughly the same thickness.
Place a ball of ganache into the center and wrap it with the mochi. Pinch the seams together with your fingers to seal the mochi shut, and then roll the chocolate-filled mochi in cocoa powder to keep it from sticking.
Adjust the shape of your chocolate mochi ball as needed and repeat with the rest of the mochi and ganache.