Poke potatoes all over with a fork. Bake in preheated 400°F oven for 1 hour or until very tender (err on the side of overcooking). Beat together egg, egg yolk and salt; add nutmeg and/or parmesan, if using.
Once potatoes are cool enough to hold (but still very warm), remove the skin and pass through a potato ricer or a food mill fitted with a small-hole disc. Spread potatoes on a clean work surface which has been lightly floured.
Drizzle beaten egg over potatoes and sprinkle with about 3/4 cup flour. Using a bench scraper, lift and turn the potatoes over on themselves, mixing in eggs and flour. Continue to scrape and turn until it forms a soft mass. Add more flour as needed until it becomes a soft but workable dough and stops being sticky. Use the smallest amount of flour necessary to make a smooth, cohesive mass.
Shape the dough into a thick log. Scrape clean your work surface. Remove a quarter of the log, keeping the remainder covered with a damp towel. Lightly flour the work surface, and roll the small piece into a rope 1/2-3/4" thick. Roll from the center out to the edges; do not stretch or tear the dough.
Cut the rope into 1" pieces. Optionally, roll each piece over the back of a fork to form ridges. Transfer to a cookie sheet and cover with a damp towel while working with the remaining dough. Cut off 1/3 of the remaining dough and repeat last 2 steps, then cut remaining mass in half and repeat twice more.
Gnocchi can be frozen at this point - freeze on cookie sheet; when solid, transfer to airtight storage for up to three months.
Uncooked gnocchi can be held in refrigerator up to 2 hours; any longer and they will get mushy.
Cook gnocchi in heavily salted boiling water; if cooking the entire amount, divide into two batches. Cook 1-2 minutes, until they float. Serve immediately with a simple sauce, or shock in ice water, then toss with a little olive oil and refrigerate until needed. Reheat in boiling water, or saute until browned and a little crispy.