Matcha is made from steamed tea leaves that are ground to a powder.
It is prepared by placing 1 teaspoon of matcha powder into a ceramic bowl (Kli Sheni) and taking water that is kept at 80°C (176°F) in a Kli Rishon and pouring 30 ml down the side of the bowl, not directly onto the powder whisking the matcha and the adding 90 ml of water pouring it down the side of the bowl.
Would this be, halachically speaking, mutar on Shabbat? I think this would be a case of ain bishul acher bishul, the steaming being the first bishul.