The secret to making a good Bavaroise is making sure you fold the whipped cream, the egg whites and the mousse at the correct setting point. The key is preparation and timing. Only whip the egg whites in the last minute or they will collapse. Do not let the mousse mixture over-set before folding and it should be just before setting point when pouring in to molds. That is before it gets lumpy. If this happens put the mixture over hot water in a bain-marie just for a moment to get it back.Bavarian cream or Bavaroise as better known is a cold dessert of egg custard stiffened with gelatine, mixed with whipped cream and sometimes with fruit purée or other flavours, then set in molds or used as a filling for cakes and pastries.
No one is sure about the origin of Bavarian cream, but during the late 17th and early 18th centuries many French chefs worked at the court of the Wittelsbach Princes, a German family that ruled Bavaria from the 12th century to 1918. The famous French chef Marie-Antoine Carême (1783-1833) gives recipes for it in the early 18th century. There are many variations, flavored with chocolate, lemon, kirsch, etc.
So the most likely origin is that the French chefs working for the Bavarian rulers learned something either the same or very similar while working in Bavaria, and when they returned to France continued to make it, and called it Crème Bavaroise which means Bavarian Cream.