Heat oven to 425 F. Scrub and pat dry potatoes. Rub with olive oil and sprinkle generously with salt. Prick several times with a fork then place on baking sheet. Bake about an hour until a knife inserts easily. Remove from oven and let cool about 10 minutes.
Slice each in half and scoop potatoes out of skins into a bowl. Reduce oven temperature to 375 F.
Mash potatoes with the butter until smooth. Add yolks, salt, pepper and nutmeg and mix very well. Spoon mixture into a piping bag fitted with a star tip and line a baking sheet with parchment paper. Pipe the potato mixture in desired shapes and return to the oven for about 15-20 minutes, or until slightly browned. Serve immediately. Store leftovers in the refrigerator.