1In a large bowl, beat the butter and sugar with an electric mixer, about 2 min. Slowly add the eggs and yolks. Beat for 1 min. Mix in the lemon juice. The mixture will look curdled, but it will smooth out as it cooks.
2In 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. (Once it was completely melted, mine thickened and reached 170º in just a few minutes, so watch it closely.) It should leave a path on the back of a spoon and will read 170°F on a thermometer. Don’t let the mixture boil.
3Remove 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. Covered tightly, it will keep in the refrigerator for a week and in the freezer for 2 months.