In a double boiler, combine the eggs, lemon juice, zest, sugar and salt. Whisk them constantly as the water comes to a simmer until the mixture begins to thicken, about 10 minutes.

Just as it thickens, switch to a wooden spoon or spatula and continue to stir, making sure to scrape down the sides of the bowl. Remove the bowl from the heat and continue to mix for 2-3 minutes as it cools.

Add butter and incorporate completely. Store in a glass jar or bowl, fully covered for up to 2 weeks.