Preheat oven to 375 degrees F.
Wash and pat dry potatoes. Bake in oven for 1 to 1 1/4 hours. Cool slightly. Peel and mash very well by hand or use ricer.
While potatoes (mashed) are still warm, pour flour into a high lump and make a well in the center. This can be done in a large bowl, on a board, or on the counter top).
Mix mashed potatoes with egg yolk, if using.
Place mashed potatoes into flour well.
With your hands, work flour into the well. Knead dough just for a few seconds (dough will be sticky) and form a ball.
Lightly flour your table or working surface.
Take a large piece of dough and roll by your palms to form a rope, almost an inch in diameter. With a sharp knife, cut pieces off (a few at a time), about an inch in length.
With your thumb (flour if needed) shape pieces like ears. Line pieces (not touching) on the floured surface you're working on.
Continue until all potato dough is used.
Bring large pot of water (a little more than 3/4 full) to a rapid boil. Add a palmful of salt and the gnocchi (enough not to overflow the pot). Remove with a large slotted spoon when they surface and place in a large serving bowl.
Continue this process until all gnocchi are cooked.
Toss with tomato sauce.
OR Toss with 5 tbsp butter and 1 cup freshly grated parmigiano.
OR Toss with 5-6 tbsp butter, a handful of finely chopped fresh sage leaves, and 1/2 cup parmigiano.