1. Put the dry ingredients into a mixing bowl: Flour, sugar, baking powder, zest and butter (treat butter as 'dry' for our purposes). Mix until you have an even sandy consistency.
2. Measure out the milk in a jug, add the egg and whisk well. Drizzle into the flour whilst mixing, until you have a smooth batter.
3. Spoon evenly into 12 cupcake cases and bake at 170°C for 20 minutes until bounce back to the touch. Cool on a wire rack and in the meantime make the icing by mixing all the ingredients together for several minutes until light and fluffy.
4. It is hard to over mix icing, so just keep going until it is really pale and fluffy. Add more milk/lemon juice or icing sugar to change the consistency. You should aim for something that holds its shape without being too stiff.
Pipe the icing on top of the cakes using a reusable piping bag and large star nozzle and decorate as required.