Bake oven in a 375.F oven until very tender.
When cool enough to handle, peel and rice potatoes into a large bowl. Add in the salt, egg yolk, and flour, mixing together by hand until dough begins to stiffen.
Transfer mixture to a work surface and knead lightly, gradually adding up to 1/2 c. more flour as needed if dough is very sticky. Continue working until dough is soft, pliable, and still a bit sticky.
Divide dough into portions the size of your fist. Working with one portion at a time, roll the dough back and forth on the surface to form a coil the thickness of your thumb. Cut each coil into 1" pieces and set aside. Continue with remaining portions.
Using a textured board or a fork, shape the gnocchi by taking a single piece, gently press the dough into the surface using a rolling motion so that the dough is imprinted with the texture. Continue with remaining dough. Place on a floured surface and use soon.