In a mixer, cream butter, lemon zest, and sugar until smooth. Beat in eggs one at a time, mixing after each addition.

Stir in lemon juice and salt-- mixing until smooth.

Transfer mixture into a sauce pan and cook over medium-low heat, stirring constantly.

After about 10 minutes the mixture will start to thicken and become glossy. Continue stirring and cooking until it's thick enough to stick to the back of a spatula when you run your finger through it. Remove from heat. Do not boil.

Let cool, cover and refrigerate. Last up to a week in the fridge, 1 month in the freezer.