“Puto bumbong is a type of Filipino purple rice cake which is prepared by steaming ground purple rice mixture inside a bamboo tube. The tube is referred to as “bumbong ng kawayan” in Filipino. This might have been the reason why this was named as such.
Puto bumbong has been associated with the Christmas season because it is mostly sold outside the church during the traditional simbang gabi. The puto bumbong vendor can be observed steaming a powdered concoction inside a bamboo tube using an odd shaped steamer. The steamed rice cake is then placed on a piece of pre-cut banana leaf and then topped with margarine (or butter), freshly grated coconut, and muscovado sugar.“
Source : https://panlasangpinoy.com/2017/12/19/puto-bumbong-recipe/