20th Century Cafe, that retro-fabulous corner bakery in Hayes Valley, is publishing its first and definitive cookbook today. The cafe is known for its old-world European pastries, inspired by the grand cafe tradition in Vienna, Budapest, and Prague. Baking at the 20th Century Cafe is not for the faint of heart. It’s an advanced baking book, from a professional pastry chef, assiduously detailing methods for assembling ten-layer tortes and stretching strudel. But for cake diehards, it’s a rare treat, digging