Steamed lemon puddings

1. In a small bowl, using an electric mixer, beat butter and sugar together until creamy.
   
2. Add eggs one a time, beating after each addition, until mixture is pale and fluffy. Fold in flour, milk and zest.
   
3. Lightly grease and line the bases of 6 x ¾-cup metal moulds or ramekins. Place one piece of lemon in the base of each mould.
   
4. Divide pudding mixture evenly between moulds. Cover tightly with greased, pleated foil. Tie with kitchen string to seal.
   
5. Place in a large saucepan. Pour in enough boiling water to come halfway up the sides of the moulds. Cover the saucepan with a tight-fitting lid.
   
6. Bring to the boil on high. Reduce heat to low and simmer for 40 minutes. Remove from pan.
   
7. Remove string and foil from puddings. Turn onto serving plates and serve with pouring custard and extra lemon slices.