In a small mixing bowl whisk together the lemon juice,egg yolks and sugar until the sugar is mostly dissolved.

Pour the mixture into a nonreactive saucepan. Bring to a simmer over medium heat. Reduce the heat to low and continue to cook, stirring constantly with a wooden spoon, until the mixture thickens and coats the back of a spoon (this happens at about 170 degrees F) -- take sure not to over cook the curd or the eggs will curdle. Remove from the heat and immediately transfer to a clean, heatproof, nonreactive bowl. Stir in the butter, 1 tablespoon at a time. Cool the curd to room temperature, then transfer to a resealable container and refrigerate until ready to use. The curd will continue to thicken as it cools. The curd can be stored in the refrigerator for up to 3 weeks in an air-tight resealable container.