Can you tell me the story of how Sai Baba fulfilled Laxmichand's desire for a specific sweet?

Certainly. The story of Laxmichand and the khaja is a wonderful example of Baba's omniscience. As recounted in Chapter 28, a devotee named Laxmichand was visiting Shirdi. After having some khaja (a sweet) on the first day, he developed a craving for more. On the third day, Baba miraculously knew of this unspoken desire. He instructed his devotees to bring him a plate full of khaja for the naivedya, or food offering. Later, he told everyone it was an auspicious day and he felt like having khaja, ensuring that bowls of the sweet were brought and served to all, much to the astonishment and delight of Laxmichand, whose attachment to Baba was then fulfilled.