Cream together the butter and sugar, using an electric mixer on medium, for about two minutes.
Slowly add the eggs and yolks and beat for another minute.
Mix in the lemon juice.
Transfer to a medium saucepan with a heavy base and cook over low heat until the butter melts. The mixture should lose its curdled look and become smooth.
Increase the heat to medium and cook for about 15 minutes, stirring constantly, until the temperature reaches 170 degrees F and the curd thickens. When the curd is ready, it should coat the back of a spoon.
Be careful not to boil or to cook the eggs. (You may want to heat the lemon curd over a double boiler to prevent cooking the eggs.)
Remove from the heat. Add the lemon zest and stir to incorporate.
If necessary, strain through a fine mesh strainer. Discard solids.
Transfer to small glass jars and allow to cool and thicken.
Refrigerate for up to a week or freeze for up to a year.