Pre-heat the oven to 160°C (320°F).
Cream the butter and sugar until smooth. Beat in the egg.
Sift the flour and baking powder, and fold into the batter. Add the lime juice, followed by the milk, stirring until well combined.
Fill eight cupcake cases about half way.
Place in the lower part of the oven, and bake for around 15-17 minutes. They areready when an inserted skewer comes out clean.
Leave on a wire rack to cool.