Sift together the flour, baking soda, and green tea powder and set aside. In a large bowl, beat the butter until light. Add the sugar and beat some more until light and fluffy. Add the dry mixture and the milk. Stir with a spatula or wooden spoon until the dry ingredients are just incorporated (don't overmix). The batter will be somewhat lumpy. Divide the batter among the muffin tins (I used those foil liners. If you don't have any liners, grease the tins), about 2/3 full. Bake for 25-30 minutes. Allow to cool on a wire rack.