Using an electric stand mixer, cream the butter until it is smooth. Add powdered sugar, water, and matcha powder, and beat on high for 1 minute until the ingredients are mixed. Scrape down sides of the bowl, check to see if the buttercream is spreadable, add water as necessary and beat for another minute. Reserve until ready to use.
Preheat oven to 350 degrees F and line cupcake/muffin tins with parchment paper liners. Process the chocolate chips in a food processor. Place over a double boiler and melt until tempered and stream-able. Warm hold until ready to incorporate.
Place cake mix, water, oil, eggs, matcha powder in a large mixing bowl and use an electric mixer to mix at low speed for 30 seconds, then beat at medium speed for 2 minutes.Stream in the chocolate and incorporate well. Pour the batter into the pans and bake for 18-20 minutes, or until cooked trough completely.When the cupcakes are ready, take them out of the oven and let them rest in the pans for a few minutes and then take them out of the pan to cool on a rack until completely cooled.