I can't understand this joke.

Pancake fruit, which is a famous snack in Tianjin. In fact, it is to wrap fried dough sticks or fruit grates on a cake coated with eggs.

You can actually understand the shape of this food. Does it look like a scene: a strip like fried dough sticks is tightly wrapped by a cake full of eggs? This kind of food is a vivid metaphor for unspeakable behavior. This is why the waiter at KFC misunderstood the boy's meaning.