In a small saucepan whisk together the egg yolks, sugar, lemon juice and lemon zest.

Then on low-medium heat, cook the lemon mixture until it boils lightly then thickens, making sure to whisk constantly so there are no lumps. This step should only take about 5 minutes.

Remove the lemon mixture from the heat and stir in the butter until it is melted and thoroughly mixed in. The curd should look creamy.

Set aside to cool. Or you can use it right away...warm lemon curd is delicious too!