Preheat the oven to 190°C (170°C Fan, Gas Mark 5). Line a cup cake tin with the Baking Cases. In a mixing bowl, beat the margarine with the sugar until pale, and creamy-light in texture.
Gradually whisk in the eggs and Vanilla Extract then sift the flour and Baking Powder on top. Add 100g of the Chocolate Chunks and using a large metal spoon, carefully fold the ingredients together until well mixed.
Spoon into the Baking Cases and bake on the middle shelf in the oven for 16-18 minutes until risen, lightly golden and just firm to the touch. Transfer to a wire rack to cool completely.
Once the fairy cakes have cooled, spoon the icing into a large piping bag fitted with a 1cm (½ inch) large closed star nozzle, and pipe a swirl to cover the top of each cake. Decorate the cakes with the remaining Chocolate Chunks to serve.