Cut butter into small pieces and put in refrigerator
In a large aluminum or stainless steel mixing bowl, whisk the egg yolks until they lighten and about double in volume
Add the sugar to the egg yolks, whisk until smooth
Add the lemon juice and lemon zest, whisk until incorporated and smooth
Bring an inch of water to simmer in a small sauce pan and place the mixing bowl over it. Keep the heat low enough to maintain a simmer and whisk the mixture for 6 to 10 minutes, until it lightens and thickens. It should coat the back of a spoon.
Take the mixing bowl off the heat and whisk in the cold butter pieces a few pieces at a time. Pour into an airtight container and place in refrigerator to finish cooling.