Preheat the oven to 200° C / 400 ° F. Take the marrow out of the bones with a small spoon. Roast the marrow in the oven for 10-15 minutes. Peel the potatoes and boil them until done. This usually takes 20 minutes.
Get rid of the water from the potato pan, and start mashing the potatoes with a fork. Add the milk, butter and marrow, and stir regularly. Add salt and pepper to taste.