makes 1½ cups; Recipe adapted from Joy of Cooking
Put eggs, sugar and lemon zest in a medium saucepan and whisk off heat for a couple minutes off until pale in color.
Add the lemon juice, butter and salt and cook over moderate heat until butter is melted and mixture begins to thicken, about 6–8 minutes (160–170° F). Pour through a sieve into a glass jar for storage. Refrigerate for up to a week.