Preheat the oven to 190°C (170°C Fan, 375°F, Gas Mark 5). Line 10 cup cake tins with brown Cupcake 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 on top. Add the Chocolate Chips and using a large metal spoon, carefully fold the dry ingredients into the eggy mixture.
Spoon into the Cupcake Cases and bake on the middle shelf of 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 American Peppermint Extract to taste.
Transfer to a piping bag fitted with a 1cm (1/2inch) star nozzle and pipe a generous peaked swirl on top of each cake. Decorate with Polka Dots before serving.