Whisk the egg yolks and sugar in a heavy bottomed saucepan. Set on low to medium heat (I set it on 4 out of 10 where 10 is the hottest temperature). Add the butter and heat slowly until the butter melts.
Add the lemon zest to the egg yolk mixture. Once the butter has melted, add the lemon juice. Stir (using a whisk or a spoon) as it thickens, about 15 minutes.
Remove pan from the heat, put the lid on it and let it thicken further from there. When cool, placed in a clean, sterilized jars.
Store in refrigerator for up to 3 weeks