6 tbsp unsalted butter, softened at room temperature
1 cup sugar
2 large eggs
2 large egg yolks
2/3 cup fresh lemon juice
1 tsp grated lemon zest
In a large bowl, beat butter and sugar with an electric mixer for about 2 minutes.
Slowly add the eggs and yolks and beat for a further minute. Mix in lemon juice. The mixture may look curdled, but will smooth out as it cooks.
In a medium, heavy based saucepan, cook mixture over low heat until it looks smooth.
Increase heat to medium and cook, stirring constantly, until the mixture thickens, about 15 minutes. It should leave a path on the back of a spoon and will read 75 degrees Celsius on a thermometer. Take care not to let the mixture boil.
Remove the curd from the heat and stir in the lemon zest. Transfer the curd to sterilized jars or a bowl (if using 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.