Lemon Curd

An old favourite that I use as the filling for lemon meringue pie or lemon tartlets. An indulgent treat by the spoonful.
Ingredients
100 gm unsalted butter, chopped
200 gm castor sugar
2 lemons (juice and rind)
4 egg yolks, beaten

Special utensils
A double boiler, or bowl that can sit over a pot of boiling water without touching the water.

Method
Put butter and sugar in a stainless steel bowl and place over a pot of boiling water. Mix with a wood spoon until butter has melted.
Stir in lemon rind, lemon juice and egg yolks.
Continue to stir until mixture thickens enough to coat the back of a wooden spoon. (Around 20 minutes).
When thick pour immediately into a clean bowl or jar.

Tips
My mother always told me that if perchance you curdle the mixture, quickly pour a tablespoon of boiling water into the mixture. I've done this heaps of times and it generally works.