Line a 12-section bun tray with paper cake cases. Put the mint leaves in a heatproof bowl, cover with boiling water and leave for 30 seconds. Drain and pat dry on kitchen paper. Put the leaves in a food processor with the caster sugar and process until the mint is finely chopped.
Transfer the mint sugar to a bowl and add the butter, eggs, flour and baking powder. Beat with a hand-held electric whisk for about a minute until light and creamy.
Stir in 100 g (3½ oz) of the chocolate and then divide the cake mixture between the paper cases. Scatter with the remaining chocolate.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until the cakes are risen and just firm to the touch. Transfer to a wire rack to cool. Lightly dust with icing sugar.