Place the bones, cut-side up, onto a baking sheet and cook the marrow until soft and slightly separating from the bone, about 15 minutes.
Meanwhile, boil salted water for potatoes and let them cook until soft, about 25 minutes.
Remove the marrow bones and keep aside. Drain the potatoes and add the melted butter and cream. Mash until smooth. Using a spoon, scoop out the marrow with the juices and add to the mash. Add the parsley and season with salt and pepper.