In a large bowl, beat the butter and sugar with an electric mixer, about 2 minutes.
Slowly add the eggs and yolks.
Beat for 1 minute.
Mix in the lemon juice.
The mixture will look curdled, but it will smooth out as it cooks.
In a medium, heavy-based saucepan, cook the mixture over low heat until it looks smooth. (The curdled appearance disappears as the butter in the mixture melts.)
Increase the heat to medium and cook, stirring constantly, until the mixture thickens, about 15 minutes.
The curd is cooked when it reaches 170°F, but you can see that it's cooked when your finger leaves a clear path on the back of a spoon. Don't let the mixture boil.
Remove the curd from the heat; stir in the lemon zest.
Transfer the curd to a bowl.
Press plastic wrap on the surface of the lemon curd to keep a skin from forming and chill the curd in the refrigerator
The curd will thicken further as it cools.
Notes
Covered tightly, it will keep in the refrigerator for a week and in the freezer for 2 months. Use a heavy-based, nonreactive saucepan — Stainless steel, anodized aluminum, and enamel all work well. Some materials, such as plain aluminum or unlined copper, will react with the acid in the lemons, discoloring the curd and giving it a metallic flavor.