In the top half of a double boiler, combine sugar, lemon zest, and egg yolks. Whisk together until well combined. Whisk in the lemon juice and the salt.
Add the pieces of butter.
In the bottom pot of a double boiler, put about 1 inch of water. Bring the water to a boil, then reduce to a gentle boil - medium heat.
Set a fine-mesh sieve over a glass bowl. Have it on close by, you'll need it soon.
Put the top of the double boiler, full of the mixed ingredients, into the bottom pot which now has simmering water in it. Cook the mixture, whisking constantly, until the butter has melted, and the mixture is thick enough to coat the back of a spoon. Do NOT boil.
Remove the pan from heat, and remove the top part of the double boiler from the bottom. Quickly pour the hot curd through the mesh sieve into the glass bowl. Use a spoon or spatula to press it through. This removes the zest, and any chunky bits of yolk.
Press plastic wrap against the surface of the curd to prevent a skin from forming on the top. Chill in the fridge for at least 3 hours.
This can be kept in the fridge for up to 2 weeks.