Preheat the oven to 190°C (170°C fan oven, 375°F, gas 5). Line 10 cupcake tins with brown paper cup cake cases. In a mixing bowl, beat the margarine with the sugar until pale and creamy-light in texture. Stir in the vanilla extract.
Gradually whisk in the eggs, then sift the flour, cocoa and baking powder. Add the chocolate chips and using a large metal spoon, carefully fold the dry ingredients into the mixture.
Spoon into the cake cases and bake on the middle shelf in the oven for 16-18 minutes until risen and just firm to the touch. Transfer to a wire rack to cool.
For the icing, place the butter in a bowl and beat until soft. Gradually sift and beat in the icing sugar and cocoa powder to make a smooth, creamy icing. Add peppermint extract to taste.
Transfer to a piping bag fitted with a 1cm (½ inch) star nozzle and pipe a generous peaked swirl on top of each cake. Decorate with polka dots before serving.