Preheat the oven to 180C/Gas 4. Grease two 20cm/8 inch round sandwich tins and line with non-stick baking paper.
Place the butter and sugar in a bowl and cream together until thick and pale. Beat the 3 eggs and add to the bowl, then sift in the flour gradually. Finely grate the zest from the 2 lemons into the bowl, and squeeze 1 tbsp lemon juice and add.
Beat together for about 2 minutes, until the mixture is light and fluffy. Spoon into the tins and spread level. Bake for 30-35 minutes or until firm and golden, and leave to cool in the tins for 10 minutes. Then turn out onto a wire rack and leave until completely cooled.
To make the icing, beat the butter with the icing sugar until light and fluffy, and squeeze 1 tbsp lemon juice into the mixture.
Spread the icing over the top of each cake using a palette knife, and sandwich together.