Heat the milk in a saucepan over a medium heat until it just begins to boil. Remove from the heat and add the teabags. Cover with clingfilm and let the mixture steep for at least 30 minutes, or longer if possible. You could even leave this overnight in the fridge.
When you are ready to make the cupcakes, preheat the oven to 180C/160C fan/gas mark 4. Line a 12-hole muffin tray with muffin cases. Remove the teabags from the milk and squeeze out any excess milk into the pan. Add the green tea powder, and stir.
In a bowl cream the butter and sugar with an electric hand mixer until the mixture is pale and smooth. Add the eggs, one at a time, mixing briefly after each addition. Sift the flours into a separate bowl and stir well. Add one-third of the flour mix to the creamed butter and sugar and beat well. Pour in one-third of the milk/tea mixture and beat again. Repeat these steps until all the fl our and milk has been added.
Carefully spoon the mixture evenly into the muffin cases, to about two-thirds full. Bake in the oven for about 25 minutes, until raised and golden brown. The cakes will have a greenish tinge to them.
Leave the cupcakes in their tin for 10 minutes or so before turning out onto a wire rack. When completely cool, ice with the green tea buttercream icing and sprinkle with a little green tea powder.