You can use your boiler or the other method of a large saucepan with water in and a heatproof glass bowl over the saucepan.
Fill the bottom pot with 2 ½ tablespoon water (2 inches), place on high heat.
Once the water begins to boil reduce to low heat to keep the water simmering.
Place the egg yolks, sugar, lemon zest, lemon juice, and salt into the top part of your boiler or heatproof glass bowl.
Using a silicone whisk, whisk until all is blended then continue whisking as the curd begins to cook.
Continuously whisking to prevent the egg yolks from curdling.
Whisk and cook until the mixture becomes thick, same texture as Hollandaise sauce, 10 minutes.
If the curd isn't thickening turn up the heat and continue to whisk.
Removing the pan from the heat.
Cut the butter into 6 separate pieces, then whisk into the curd.
The butter will melt from the heat of the curd. Pour the curd into a jar or bowl and place a piece of plastic wrap directly on top so it is touching the top of the curd.
The curd will continue to thicken as it cools down.
Once cooled the plastic wrap can be removed.
Refrigerate the curd for up to 10 days.