1. Whisk together egg yolks and eggs. Combine with sugar and lemon juice in a small, heavy-bottomed saucepan. Cook over low heat, stirring constantly, for 8 to 10 minutes, or until mixture coats the back of a spoon.

2. Remove pan from heat and stir to cool slightly. Strain through a sieve into a small bowl and add butter, a piece at a time, stirring until smooth. Stir in zest and let cool completely before using.