Diplomat cream is a delightful custard-based cream that combines whipped cream and pastry cream (also known as crème pâtissière). The result is a light, airy texture that's perfect for desserts like this berry tart. In this recipe, you start by making a custard with milk, egg yolks, sugar, and cornstarch, which is then cooled. Once chilled, whipped cream is folded into the custard, creating the diplomat cream. This cream serves as a luscious filling that complements the tangy flavors of the fresh berries beautifully, ensuring that each bite is both rich and refreshing. Utilizing diplomat cream not only enhances the tart's texture but also adds depth of flavor, making it a standout dessert for special occasions.