In a medium stainless steel or enamel saucepan (aluminum will discolor the curd) whisk together eggs, sugar and lemon zest until light in color. Add lemon juice and butter and cook, whisking, over medium heat until butter is melted and blended. Whisk constantly until the mixture is thickened and simmer gently for a few seconds. Scrape the filling into a medium-mesh sieve using a spatula and set over a bowl to strain the filling. Stir in the vanilla. Let cool, cover and refrigerate to thicken.
Adapted from From The Joy of Cooking
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