Heat the oven to 180C/fan 160C/gas 4. Line a 12 hole cupcake tin with muffin cases. Beat the butter in a stand mixer until it is soft and paler in colour, then add the sugar and keep beating until you have a light, fluffy mixture. Beat in 1 egg followed by a spoonful of flour, and then repeat with other 2 eggs, add the baking powder, a pinch of salt and the matcha to the remaining flour and fold it in. Add a tablespoon of water if the batter is too thick. Divide the mixture between the muffin cases, they should be about 2/3 full. Push a chocolate into the centre of each. Bake for 15-20 minutes, or until the cakes are golden and risen and feel firm to the touch, don’t try testing them with a skewer because of the chocolate in the centre. Cool on a wire rack.