Chocolate to use for Buche de Noel
My general rule, based on personal preference, local availability, and price, is Callebaut for milk and white chocolates, El Rey for dark.
On a related topic, I had one of the El Rey single-origin chocolate bars this weekend (don't remember the name) and it was fantastic. There was a strong citrus fruitiness to it - I am planning to save up enough money to buy some for making an orange/chocolate torte.
My Saveur recipe calls for semisweet choc. for the icing and filling, whereas bittersweet is used for the roulade (cake part).
No specific brand is called for; however, as usual, the better quality product you start out w/ the better the result IMO. I couldn't personally justify or afford using all Valrhona, so I believe I just used Ghirardelli choc. chips in my case. It still tasted good to me, but I'm sure the Valrhona would have sent me over the edge...
I like Lindt. I find the flavor very good, and it is almost always available in the supermarket. I use Lindt dark and milk chocolate all the time when I make desserts. The milk chocolate a particular temptation for me, so I only buy it if I am going to cook with it. Otherwise, I'd consume all of it within days.
When I make the buche de noel, I like to do a whipped chocolate ganache filling, so that the whole cake is chocolate through-and-through. I prefer a sweeter whipped ganache than what Beranbaum calls out for in her recipe, so I usually replace about 1/3 of the dark chocolate with milk chocolate. To my taste, this is the right amount of sweetness.