Line a 23cm x 30cm tray with baking paper and preheat the oven to gas mark 4 or 180 degrees Celsius.
Beat together the butter and caster sugar in a bowl
Gradually add the egg and self-raising flour while folding, until you have combined all the ingredients.
Add the lime zest, mix well then transfer the cake mixture tthe tray.
Bake in the oven for 25 minutes, then leave to cool.
Prepare the icing by mixing icing sugar with the juice of two limes. Spread across the cake, then sprinkle with lime zest. Leave to set, then cut into squares to serve.