Take two cooking pots and make sure the smaller one fits into the bigger one. Fill the bigger pot with water so it's just touching the bottom of the smaller pot. Put on medium heat.

In the smaller pot mix together the egg, the egg yolk, the sugar and the lemon juice.

When the mixture starts to get warm start stirring (I get the best results when using a whisk). It'll take a couple of minutes, but soon the mixture will start to thicken. When the curd coats the back of a spoon you can take it off the heat and add the lemon zest

Cut the butter into chunks and whist it in one at a time.

Use the curd to fill cakes, spread it on scones or pur it over icecream.