In a medium-sized saucepan, whisk together eggs and sugar, then stir in lemon juice and zest. Place saucepan over a low flame, and constantly stir mixture until it thickens and coats the back of a spoon. Remove from heat and let mixture cool to just warm, then strain through a mesh sieve into a medium bowl. Gradually add in the butter one piece at a time, whisking until thoroughly combined. Store covered in the refrigerator at least 2 hours.