In a stainless steel bowl placed over a saucepan of simmering water, whisk together the eggs, sugar, and lemon juice until blended. Cook, stirring constantly (to prevent it from curdling), until the mixture becomes thick (like sour cream or a hollandaise sauce) (160 degrees F or 71 degrees C). This will take approximately 10 minutes. Remove from heat and immediately pour through a fine strainer to remove any lumps. Cut the butter into small pieces and whisk into the mixture until the butter has melted. Add the lemon zest and let cool. The lemon curd will continue to thicken as it cools. Cover immediately (so a skin doesn't form) and refrigerate for up to a week.
Makes 1 1/2 cups (360 ml).
If you want a lighter lemon curd whip 1/2 cup (120 ml) of heavy whipping cream and fold into the lemon curd.