Puto bumbong is a Filipino purple rice cake made by steaming a bamboo tube filled with pulverized purple rice. In Filipino, the tube is referred to as "bumbong ng kawayan." It's possible that this is why it was given that name.

Because it is usually sold outside the church during the customary simbang gabi, puto bumbong has been connected with the Christmas season. Using an oddly shaped steamer, the puto bumbong merchant can be seen boiling a powdery mixture within a bamboo tube. The steamed rice cake is then placed on a banana leaf that has been pre-cut and topped with muscovado sugar, freshly shredded coconut, and margarine (or butter).

More Info: panlasangpinoy.com