Zest 2 lemons
Cream butter in mixer.
Add sugar and lemon zest.
Mix together until well blended.
Then add 4 eggs, one at a time, and mix until well blended.
Add lemon juice, using the 2 lemons you zested, plus 1 more (about 1/2 cup lemon juice) and salt and mix well. Don't be tempted to use bottled lemon juice. The real lemon juice is the key to good lemon curd.
Be sure and scrape the sides of the mixing bowl occasionally to make sure it is all getting incorporated.
Using a glass bowl over a saucepan as a double boiler, pour your mixture in and cook over low heat, stirring constantly, until thickened (about 10-12 minutes).
Be sure and use a glass bowl and a spoon that isn't metal. The metal can give the lemon curd a strange taste.
If your lemon curd isn't thickening, at this point, turn the heat up and add 2 tablespoons butter. Using a whisk, stir constantly until thickened.
Just remember, as it cools, it will thicken a bit too.
Remove from heat and let cool.
Pour into glass airtight containers.