Prunes, or dried plums, are a delicious, often underrated baking ingredient in the United States. In France, the fruit is used in sweet and savory recipes, including this one for a popular dessert filled with frangipane, or buttery almond cream, and plenty of plumped fruit, baked together until the filling turns a golden brown on top and the prunes are as sweet and tender as caramels. You could use a store-bought pie or tart shell, slightly parbaked before filling it up, or follow a recipe…