Peel only half of the potatoes, and cut them in half. Place the potatoes in a large pot and cover with water. The water should be about an inch higher than the potatoes. Add a dash of salt to the pot. Bring to a boil, add salt and simmer until tender, about 15 minutes.
Drain potatoes well. Put then back in the pot and cook over low heat to evaporate the rest of the water. Mash in the butter. Add heavy cream a little at a time until desired consistency reached. Fold in almost all of the feta, reserving some for the top of the dish. Season with salt and pepper, to taste.
Garnish with the remaining feta cheese and serve immediately.