Preheat the oven to 150°C/Gas Mark 2.
Grease and flour 2 Tala Mini Decorative Cake Pans and set aside.
Place the margarine and caster sugar into the mixing bowl and cream with a wooden spoon.
Add the egg and mix thoroughly, adding some of the flour now and then if the mixture starts to look lumpy.
Add the rest of the flour and mix until smooth. Finally add the zest and mix briefly to distribute it evenly through the mixture.
Divide the mixture equally between the two tins and bake for 15 minutes. Turn if necessary, then bake for a further 15 minutes or until the cakes spring back when pressed gently in the centre.
Turn out the cakes onto a wire rack and leave to cool.
While the cakes are cooling, prepare the icing by mixing the lemon juice with the icing sugar. Pour the icing over the cooled cakes so that it drips down the sides, then sprinkle with the reserved lemon zest.