• 1/2 cup lemon juice
• 6 egg yolks
• 1 cup sugar
• 4 teaspoons freshly grated lemon zest
• 1 stick (1/2 cup) cold unsalted butter , cut into pieces

Combine the lemon juice, egg yolks and sugar in a nonreactive saucepan, and whisk until smooth. Cook over medium heat, stirring constantly with a wooden spoon until hot, about 10 minutes. As the lemon mixture gets hot, it will begin to thicken. Once thick (it should coat the back of the spoon), reduce the heat to very low and cook for an additional 5 to 10 minutes, stirring continuously.

Remove the saucepan from the heat and strain the mixture into a bowl. Stir in the lemon zest and butter until the butter has melted and is incorporated into the lemon mixture. Store in an airtight container in the refrigerator.