Make french macaron shells using your favorite recipe. I used this recipe.
Beat butter on high for 2-3 minutes until pale and creamy. Add powdered sugar and beat until smooth. Add cream, vanilla, green tea powder, and a tiny bit of green gel food coloring and beat until smooth.
Fit your piping bags with a large round piping tip. Pipe a dollop of green tea buttercream on to one macaron shell. Gently cover with another macaron shell.