Zest lemons, being careful to avoid the white pith.
Cream the butter and beat in the sugar.
Add the eggs, one at a time.
Add the lemon juice, lemon zest and salt.
Mix until combined.
Pour the mixture into a 2-quart saucepan and cook over low heat until thickened about 10 minutes, stirring constantly.
The lemon curd will thicken at about 170 degrees F, or just below simmer.
Remove from the heat and cool or refrigerate.