Curds are fruit-flavored liquids that are thickened by the addition of eggs. (The proteins in the eggs coagulate when heated.)
fresh lemon juice
In a medium saucepan melt the butter over medium heat.
Add 2 cups sugar, 3 T lemon zest, and 3/4 cup lemon juice to the melted butter. Heat and cook until the sugar melts, about 2 - 3 minutes.
Beat the 4 eggs in a medium bowl until frothy.
Slowly add one cup of the lemon juice/butter mixture to the frothy eggs and stir. Then pour the lemon juice/butter and eggs into the saucepan.
Cook and stir 7 - 9 minutes until it thickens.
Use for filling for pies, rolls, as a spread on bread, or a sauce. It is fantastic on English muffins. Makes one quart.