In a double broiler (or small stainless steel bowl placed over simmering water) whisk eggs, sugar and lemon juice until well blended. Cook over simmering water and whisk constantly until mixture thickens, about 10 minutes.

Remove from heat and immediately strain through a fine mesh sieve into a small bowl to remove any lumps

Stir in butter and whisk until melted. Stir in lemon zest. Let mixture cool. Cover immediately and store in refrigerator for up to 1 weeks.