Are you suggesting there are guests out there who, looking for something sweet, would eat a poop brownie, but if that were not available, would not eat any other type of sweet or baked goods (such as plain, square brownies)? You are always the one arguing about cannibalization of business; Ordinarily such a view overlooks too many intangibles, among other problems, but in this case where the primary difference in brownies is just the shape and presentation of the product, I would tend to think most people could probably find an alternate dessert they'd like about as well (and based on taste reports, probably better).
Certainly some people would be attracted by the novelty of the item, but that same argument also supports a greater selection of specialty merchandise available in the parks (various t-shirt designs, attraction & resort-specific items, etc.).
