75 g (3 oz) mint-flavoured plain dark chocolate, chopped
Line a 12-hole nonstick muffin tin with paper cupcake cases or lightly grease. Place the cocoa powder in a small bowl, then stir in the boiling water and peppermint extract until smooth. Set aside.
Place the butter, sugar, flour, baking powder, eggs and milk in a large bowl and beat together with a hand-held electric whisk until pale and creamy. Scrape in the chocolate paste and beat until smooth, then stir in the chopped chocolate.
Spoon the mixture into the prepared muffin tin and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 12–14 minutes until risen and firm. Transfer to a wire rack to cool slightly, then serve warm.