Bring a medium pot of salted water to a boil. Drop the cubed potatoes into the water and cook for about 15 to 20 minutes, until the potatoes are very tender. Drain in a colander and set aside. Add the potatoes to a ricer and push through, into a a pot (you can use the same one you boiled the potatoes in).
If you don’t have a ricer, you can use a potato smasher. Repeat with the remaining cubes of potato. Mix in the creme fraiche, black pepper and salt. This can be made the day ahead and reheated by adding a splash or two of milk. Transfer to a serving dish, make pretty swoops in the top and then pour melted butter atop the potatoes and garnish with dill.