She prepares cakes for the smaller children like normal. But for Jem, she offers him a slice of the larger cake.
This symbolizes Jem's growing into the adult world. Maudie takes this opportunity to recognize him as an adult. Take a look at the text:
There was a big cake and two little ones on Miss Maudie's kitchen table. There should have been three little ones. It was not like Miss Maudie to forget Dill, and we must have shown it. But we understood when she cut from the big cake and gave the slice to Jem.
As we ate, we sensed that this was Miss Maudie's way of saying that as far as she was concerned, nothing had changed.
Something is changing, Jem is growing up and his understanding of the world's pain is becoming more and more clear. However, from Maudie's perspective, her ability to be a friend, to recognize the kids' needs, and to tell them the truths about their father that they don't have a mother to tell them will never change. This moment was one in which she had chosen to reveal some tough truths about Atticus and she wanted Jem to hear her:
"I simply want to tell you that there are some men in this world who were born to do our unpleasant jobs for us. Your father's one of them."
"Oh," said Jem. "Well."
"Don't you oh well me, sir," Miss Maudie replied, recognizing Jem's fatalistic noises, "you are not old enough to appreciate what I said."
Jem was staring at his half-eaten cake. "It's like bein' a caterpillar in a cocoon, that's what it is," he said. "Like somethin' asleep wrapped up in a warm place. I always thought Maycomb folks were the best folks in the world, least that's what they seemed like."
"We're the safest folks in the world," said Miss Maudie. "We're so rarely called on to be Christians, but when we are, we've got men like Atticus to go for us."
Miss Maudie is a character who recognizes a teachable moment and capitalizes on it. This was one of those times when she could present Jem with the message that he is growing up, and she had words he needed to hear... and she used a grown-up piece of cake to do it.