Preheat the oven to 160/325/gas 3. Grease and line the bottom of a 6 inch cake tin.
Cream together the butter and sugar until it is light and fluffy.
Add in the beaten eggs a little at a time and when they are all used up, add in the dry ingredients. Gently fold in with a large spoon or silicone spatula until no dry ingredients are showing.
Bake for 1 1/4 hours or until the top is golden brown and a skewer comes out clean.
Let cool thoroughly before icing.
The Lime Icing
Roll the limes on your work surface to loosen up the juice before zesting the skins. Put the zest to one side.
Juice the limes and mix into the icing sugar, adding a little cold water if necessary to make a thick pouring consistency.
When the cake is perfectly cool, spread the icing over the top and sprinkle with the lime zest to decorate.