To make ahead, tightly cover the completely cooled tart and refrigerate up to 3 days. To freeze, tightly wrap the completely cooled tart and freeze up to 2 months. About 1 hour before serving, unwrap frozen tart and let stand at room temperature to thaw.
To show off this sinfully rich dessert, add a toasted walnut half to each serving. To toast 12 to 16 walnut halves, cook in an ungreased heavy skillet over medium heat 5 to 7 minutes, stirring frequently until nuts begin to brown, then stirring constantly until nuts are light brown.