1. The Aztecs were not making chocolate cakes. One reason is that they didn't knew how to make it, and another reason is that they didn't had everything needed for making a chocolate cake even if they wanted to and new how. The majority of the ingredients were on disposal, especially the chocolate, which was made out of the cocoa beans that were in abundance. The Aztecs though didn't seemed to be very fond of using the chocolate to make sweet foods and drinks, but instead they were using it for making spicy drinks.
2. The ingredient that the Aztecs would have been missing for making a chocolate cake is the milk. The Aztecs, as all other native people from the Americas, didn't had any domesticated animals that were providing them with milk. In fact, the Aztecs were not even aware that they can get milk from certain animals in sufficient amounts so that they can use it in their daily diets. Also, it has to be mentioned that the Aztecs, as the other people from the Americas were actually lactose intolerant as their digestive systems were not able to process milk.