Lemon Curd

In bowl or top of double boiler, whisk eggs until smooth. Whisk in sugar, zest and lemon juice. Place over small pan of simmering water and cook over low heat, stirring constantly with wooden spoon until thick and pale yellow, 7 to 10 minutes. Stir in butter, one tablespoon at a time, until thoroughly combined and smooth. Set bowl over ice, stirring occasionally, to cool. Cover with plastic wrap touching the curd's surface and store in refrigerator up to 5 days.