I was going to say it is le Grand Cru at the Pâtisserie des Rèves. What a wonderful combination of bitter chocolate flavor and texture. Of course, it may not technically be a holiday pastry, but I don't really care.
But then on my way home from the Marché Poncelet, I passed a Maison du Chocolat. I have always loved their Salvador tarte. It's a chocolate délice-type layer cake with raspberry purée in the filling. Today I found they had replaced it for the season with a mini-Bûche Salvador. The combination of very soft glacage all around, and layers inside with dark mousse, ganache, the raspberry purée, and fresh raspberry, plus the little chocolate sponge on the bottom, is just wonderful. It gets my vote as "my favorite." I was going to take a photo. Too late.
I have to mention the Tarte Fine Porcelana from Pierre Hermé. This is another great chocolate dessert, again perhaps not just a holiday season contender. Unfortunately, the fineness of the chocolate was a bit overwhelmed in the version that included raspberry purée.