Add the lemon juice, zest, sugar, salt and egg yolks to a double boiler and whisk
Stir constantly till the mixture thickens and coats the spoon (SEE NOTES)
Turn the stove off, add in the butter and stir till it melts
Spoon the mixture into the jars and cool
Lemon Zest: The lemon Zest adds a depth of flavor and gives the curd it's true citrusy smell. I leave mine in as it gets cooked and the bits don't bother me. If you prefer your curd smoother you can run it through a fine mesh strainer before bottling it to remove the bits.Lemon Juice: Fresh squeezed lemon juice is a must when making curd. The bottled variety has preservatives that interfere with the taste and your results will be sub par.Is Your Curd Ready: The best and timeless test to determine if your curd is ready is to see if the curd coats your spoon. Then run a finger down the back of the spoon and if the line down the middle remains clear your custard is thick enough.