Mix shiratamako, cocoa powder, sugar, and water until well combined and no dry lumps of flour remain.
Transfer batter to a microwave safe dish lined with a wet dishcloth. Fold the ends of the dishcloth over the batter, cover with a microwave cover or plastic wrap and microwave on high for two minutes or until mochi dough is cooked through.
Sprinkle your work surface with cocoa powder to prevent the mochi from sticking.
Set your mochi onto the work surface and cut into eight equal pieces.
Roll or pat each piece into a flat circle about three inches in diameter, and let them cool to about room temperature.
Using a spoon or 2 tbsp cookie dough scoop, scoop balls of chocolate filling directly into the center of your mochi dough circle. Quickly but gently wrap the mochi dough around your filling to form a ball and pinch the edges shut. Dust with more cocoa powder to set and prevent sticking to the surface or to each other.
Repeat this process with the remaining seven mochi pieces. You will have leftover chocolate filling which you can use as a dip or frosting. Daifuku should be eaten the day it is made but leftovers can be kept in an airtight container in a cool room for one more day.