This recipe uses a double boiler pan. If you do not have a double boiler pan, use a heatproof bowl and set it atop a slightly smaller saucepan.  See image. Do not let the bottom of the bowl touch the simmering water inside of the pan. 

Add approximately 2 or 3 inches of water to the bottom of a medium sized double boiler pan or saucepan. Place on stovetop and set heat to medium low to create a gentle simmer. Don't forget to check the water from time to time as you're cooking. It can evaporate fast. 

Whisk the egg yolks in the top of the double boiler pan or heatproof bowl. 

Add in the sugar and whisk to incorporate. Then add the lemon juice and lemon zest. 

Place the pan or bowl on top of the double boiler or saucepan, and cook over gently simmering water.  

Add the butter one slice at a time. Whisk until each piece is melted before adding the next. 

Stir regularly until the curd thickens. This will take approximately 10 to 15 minutes.

Strain curd through a mesh sieve to remove lemon zest and any lumps. 

Refrigerate for 2 hours before serving.