Which food item is ideal for bacterial growth?

Bacterial growth doesn't mean that they grow in size, it means they increase in number by cell division. There are many factors that effects the bacterial growth, like temperature, pH, water, oxygen, and food.
Food items like bread, milk, fish, baked potatoes, raw chicken, cooked corn, these are the best for bacterial growth.

Brownies are good but they're messier than cookies. sugar cookies are also fun and more versatile to decorate. especially cut out cookies.