• Melt the chocolate in a bowl suspended over a pan of hot, but not boiling water. Keep the bowl as it will be needed again.
  50 g white chocolate
• Meanwhile, cream the butter and sugar together until light and fluffy.
  80 g unsalted butter, 125 g golden caster sugar
• Beat in the chocolate, followed by the egg.
  1 large egg
• Sift in the dry ingredients, discarding any large pieces of bran left in the sieve and mix.
  125 g flour, 1 tsp baking powder, ⅛ tsp bicarbonate of soda (baking soda), 2 tsp matcha green tea powder
• Stir in the sour cream.
  3 tbsp sour cream
• Spoon the mixture into 12 cupcake moulds lined with cases and bake at 180℃ (350℉, Gas 4) for 20 minutes when the cakes should be well risen and golden.

Matcha Buttercream Icing

• Melt the chocolate, in the same bowl used previously, over a pan of hot water and leave to cool a little.
  50 g white chocolate
• Cream the butter and icing sugar until very soft.
  80 g unsalted butter, 160 g golden icing sugar
• Add the matcha powder and cream some more.
  2 tsp matcha green tea powder
• Beat in the chocolate, followed by the cream.
  2 tbsp double cream (heavy cream)
• Spoon onto the cupcakes and top each one with a white chocolate button, if liked.