1. In a small saucepan, whisk together egg yolk and egg until combines. While whisking, pour in sugar, lemon juice and zest, and continue whisking until creamy and well incorporated, about one minute.
2. Place over medium heat, whisking constantly, until the custard thickens and can coat the back of a wooden spoon, about 8-10 minutes.
3. Remove the pan from the heat, and stir in the butter, one cube at a time. Strain into a bowl or jar, cover surface with plastic wrap and chill in the refrigerator for at least one hour.