Cream unsalted butter in a mixing bowl with an electric mixer on medium speed until soft and creamy. Gradually beat in sugar, about 1/3 cup at a time, beating until the mixture is fluffy.
Beat in eggs, beating well after each egg; mix in flour and green tea powder until thoroughly combined.
Spoon batter into the prepared muffin cups, filling them about 3/4 full.
Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out with moist crumbs or clean, 20 to 25 minutes.
Allow cupcakes to cool in muffin tins for 15 minutes before removing to finish cooling.
