The yeasty heart of the Daniel Boulud empire is hidden at the end of an East Village alley, through an unmarked door, and down a long, brightly-lit corridor. There, amid a phalanx of stainless steel ovens, mixers, and other machines, genial master baker Mark Fiorentino and his team of assistants turn out a dizzying array of breads for Boulud's half dozen restaurants. More