In a small saucepan, combine the eggs, egg yolks, sugar, lemon juice, lemon zest, and salt. Whisk the mixture thoroughly until smooth.

Set the saucepan over low heat and continuously whisk as the mixture cooks. After approximately 10 minutes, it should thicken enough to coat the back of a spoon. Initially, the surface may foam, but this will subside as it reaches the desired consistency.

Remove the saucepan from the heat and immediately stir in the cold butter, one cube at a time. Continue stirring until all the butter has melted, creating a smooth and glossy curd.

Transfer the lemon curd to a sterilized glass jar or an airtight container. Cover and refrigerate for about two hours, allowing it to cool completely and thicken further.

For optimal freshness, store the lemon curd in the refrigerator in an airtight container. It can be safely kept for up to one week.