Strain the egg yolks into a bowl. Add the sugar and lemon juice and mix well. Pour this into a medium saucepan and cook over a medium-low heat, stirring continuously until the mixture thickens and coats the back of a spoon. The cooking time will depend on the type of saucepan used so allow anything from 5 - 15 minutes. Do not allow the mixture to boil or it will be ruined. When it's thick enough remove from the heat and whisk in the butter bit by bit. Leave to cool before storing in a clean sterilised jar or a bowl. If you are planning to keep this for a while it's best to put it in a sterilised jar, but if (like in our house) this will be polished off in under a week then just a bowl is fine.
If you end up with a grainy texture you can rescue the curd by putting it through a fine sieve. Let the curd work it's way gently through the sieve and discard any solids left behind.