The cake monster is a big blob-like creature.
It's just a big gooey, sticky, and slimy creature made of flour, baking soda baking powder, etc..
The New Scooby-Doo Mysteries
When the "cake" was ready, Scooby and Shaggy opened the oven, but a large blob-like monster came out of the oven and tried to eat them, but they quickly closed it. (Scooby's Peep-Hole Pandemonium)