Crème diplomate
Crème diplomate is pastry cream lightened with whipped cream, usually stabilised with gelatine. It is noticeably lighter than mousseline while still firm enough to hold its shape in an assembled dessert.
The custard base is first blended completely smooth and chilled, and the cream whipped to soft peaks — over-whipped cream introduces grain. They are folded together in two or three additions, starting with a little cream to loosen the dense base.
Diplomate is used where lightness is wanted without losing structure: filling tarts and profiteroles, layering cakes, filling a mille-feuille. Gelatine goes in if the dessert will stand or be sliced — without it the cream eventually weeps.