Whisk together eggs and yolks, then combine with sugar and lemon juice in a small heavy-bottomed saucepan. Cook over low heat, stirring constantly, until mixture coats back of a spoon, 12 to 14 minutes. Strain into a small bowl. Add butter a little at a time, stirring until smooth. Stir in lemon zest. Press parchment or plastic wrap directly on surface of curd to prevent skin from forming. Store in refrigerator for up to 2 days, in the freezer for up to 3 months. Allow to come to room temperature before using.