In a small pot, mix warabimochiko, sugar, and water.
Place pot over medium-high heat and cook, stirring until the mixture starts to thicken.
Reduce to medium-low heat and continue stirring. The liquid should begin to clump up and form glue-like lumps. Stir vigorously to evenly cook the jelly.
When all the liquid has become jelly-like, turn off the heat and continue stirring as the residual heat finishes cooking the remaining liquid and the globs of jelly look uniform in color. It should be much less opaque than in liquid form, but it will not turn 100% translucent.
Transfer to a small, rectangular container (warabimochi should be about 3/4 inch thick) lined with plastic wrap and lightly sprayed with water. Cover and chill for 15-20 minutes or until the warabimochi is no longer hot to touch. Do not leave warabimochi in the fridge too long, or it will harden and dry out.
Sift a couple teaspoons of matcha on a clean surface and transfer cooled warabimochi on top. Roughly cut warabimochi into 3/4 inch squares and toss in the remaining matcha to cover all sides.
You can continue to chill warabimochi for up to another hour to let it completely cool before serving with matchamitsu. Warabimochi should be eaten the day it is made, but leftovers can be kept in an airtight container at room temperature as long as they haven't been covered yet in syrup.