A mille-feuille (or tompouce) is a pastry, consisting of layers of puff pastry with pastry cream in-between (see this if you don't know it).   If you buy it in a pastry store, I find that the glazed top is unique for this pastry. Recipes online tell me that it's confectioner sugar and egg whites, but I think it's something else. It's solid, yet soft. You can see your tooth print in it. It's white and sweet. I can't exactly explain how it differs from regular egg white/sugar icing, but in my opinion it does.   Does anybody have a clue what I'm talking about? Do you know what's in it? Or is it just a basic egg white/sugar icing, and is my mind playing tricks on me? 
Mille feuille (Napoleon), eclairs and petit fours, to name but a few, are definitely iced with fondant pastry - also known as poured fondant. Not a royal icing.  There are 3 types of fondant:   Pastry Fondant - known as poured fondant   Confectioners Fondant - can be interchangeable as poured fondant.   Rolled Fondant   Both poured and confectioners are identical to the naked eye. The difference being the addition of cream of tartar for the sugar inversion.  "Paul Bras" was absolutely correct in his answer.  As a semi-retired pastry chef I still make Fondant at home. No longer using a marble slab and spatulas. A modern twist, a food processor will work the hot paste 50C (120F) to a perfect smooth finish in much less time. Poured and stored in zip lock plastic bags.  