1/3 cup juice from 2 lemons
2 large eggs, plus 1 large egg yolk
½ cup sugar
2 tablespoons cold unsalted butter, cut into ½ inch cubes
1 tablespoon heavy cream
¼ teaspoon vanilla extract
Pinch of salt
Heat the lemon juice in a small non-reactive saucepan over medium heat until hot but not boiling. Whisk the eggs and yolk in a medium nonreactive bowl; gradually whisk in the sugar. Whisking constantly, slowly pour the hot lemon juice into the eggs, then return the mixture to the saucepan and cook over medium heat, stirring constantly with a wooden spoon, until the mixture registers 170 degrees on an instant read thermometer and is thick enough to cling to the back of the spoon, about 3 minutes. Immediately remove the pan from the heat and stir in the cold butter until incorporated; stir in the cream, vanilla and salt, then pour the curd through a fine-mesh strainer into a non reactive bowl. Cover the surface of the curd directly with plastic wrap; refrigerate until needed.
- At first the curd will appear thin and soupy.
- When the spatula leaves a clear trail in the bottom of the saucepan (which will quickly disappear), the curd is ready to come off the heat.
- If the curd continues to cook, it will become too thick and pasty, and a spatula will leave a wide clear trail.