In a small heavy saucepan over medium heat, whisk the eggs, sugar, lemon juice and zest until blended. Add butter; cook, whisking constantly, until mixture is thickened and coats the back of a metal spoon. Transfer to a small bowl; cool for 10 minutes. Cover and refrigerate until chilled., Just before serving, spoon lemon curd into tart shells. Garnish with raspberries, mint and/or cream if desired. Refrigerate leftovers.