3 oz. unsalted butter, softened to room temperature
1 cup sugar
2 large eggs
2 large egg yolks
2/3 cup fresh lemon juice
1 tsp. grated lemon zest
Beat the butter with the sugar until it’s light and fluffy. Slowly beat in the eggs and yolks. Beat for 1 minute more, then stir in the lemon juice. The mixture will look curdled. Do not panic.
Cook the mixture over low heat until it becomes smooth, then increase the heat to medium and cook, stirring constantly, without letting it boil, until it thickens enough to leave a path on the back of the spoon when you drag your finger through it. If you want to go by temperature, you’re looking for 170 F.
(I mixed everything in a stainless steel bowl, then put it over a pot of simmering water and whisked for 10-12 minutes.)
Remove from the heat and stir in the lemon zest. Press plastic wrap onto the surface to prevent a skin from forming and chill in the refrigerator.