Yes, you do have to continue boiling the potato beyond 15 minutes if whole. The potato is done when you can pierce it easily with a fork. However, for the best results, we recommend peeling the potato, then cutting it into 2-3 inch square chunks and boiling the potatoes for a total of 15-20 minutes. You can test the doneness of the potatoes by picking out the largest chunk with a fork and mashing it—if you can do this, the potato is fully cooked. If you boil whole peeled russet potatoes for the entire cooking process, the potato does not cook evenly and often the outer surface of the potato will disintegrate before the center of the potato is fully cooked.