Bake potatoes at 350° for 45 minutes, or until fork-tender. Let potatoes cool slightly, peel them, and then mash them in a medium-size bowl.
Gather 2 cups of mashed potatoes, and form a well in the center. Stir salt and black pepper into beaten yolk, and pour into well.
With both hands, work mashed potato and egg together; then gradually add flour until it turns into a dough. Do not overknead.
Dust a large cutting board or clean counter with flour. Cut the dough into six equal parts, and roll into ½- to ¾-inch thick ropes. Slice ropes at ½- to ¾-inch intervals.
Flour a fork and press gnocchi along tines with thumb; roll down to make ridges.
Add gnocchi to boiling, salted water; cook until they float to the top, 1–3 minutes.