This melt-in-your-mouth soft custard is a traditional English favorite. With only five ingredients, this filling makes a flavorful, easy addition to a number of desserts.
Bring sugar, zest, and juice to a boil in a heavy nonaluminum 3 1/2-qt. saucepan over medium-high heat. Remove from heat, and gradually whisk about one-fourth hot juice mixture into egg yolks; add egg yolk mixture to remaining hot juice mixture, whisking constantly until well blended.
Place saucepan over medium heat, and cook, whisking constantly, at least 10 or up to 12 minutes. (Mixture will be a pudding-like thickness.)
Add butter, in 6 batches, whisking constantly until butter melts and mixture is well blended after each addition. Remove from heat, and pour mixture through a wire-mesh strainer into a bowl. Place plastic wrap directly on warm curd (to prevent a film from forming); chill 3 hours.