Place the eggs, yolk, sugar, and lemon juice in a medium sized, heavy-bottomed saucepan and whisk together to break up the eggs. Add the butter and cook over medium-low heat, whisking frequently. When the mixture begins to bubble around the edges, lower the heat and whisk constantly until the mixture thickens and reaches 180F/82°C. (The temperature is more important than the time it takes, and the cream itself should not boil.) The curd will thicken and form a soft shape when dropped from a spoon. It will also begin to look a bit translucent. If desired, stir in the zest after removing from the heat. Let cool to room temperature, stirring occasionally to release the heat. Refrigerate for at least 6 hours or up to 1 week in an airtight container.