Combine the flour and salt, blending well. Add the milk, egg yolk, and oil, again blending well. Allow to rest for 5 minutes. Knead for about minutes and form into a ball. (This may be done in a processor.) Cover and set aside for 15 minutes or so. On a floured surface, roll out a third of the dough, into a rectangle 0.125th inch thick. Turn the dough and roll from the center, so that all the dough in the rectangle is even in thickness. Run a hand under the dough to loosen it. Dust with flour and flip over and dust with flour. With a sharp knife, cut into 4 cm squares. Place a teaspoon of mushroom filling (see the next recipe in this series) in each square, being careful not to smear the edges. Fold on the diagonal to make a triangle. Pinch together the two bottom corners, wrapping it around your thumb with the point up (to resemble small ears on a cat), making sure that the edges have bonded or the stuffing will boil out. Place on cookie sheets covered with dish towels lightly dusted with flour. Repeat with the other pieces of dough, saving the scraps for last as the dough gets a little tougher when worked. Drop 10 or dozen vushka into 6 to 8 cups of rapidly boiling water and stir once with a wooden spoon, (The wooden spoon is very important as a metal one will lower the temperature of the water.) Do NOT cover. When they float to the top, cook 1 minut, then remove with a slotted spoon to a strainer. Cool on a lightly oiled plate without crowding. Repeat until all are cooked. Cover and set aside. These may be frozen and then reheated in boiling water. (DO NOT overcook when reheating.) TO SERVE: Place 4 or 5 vushka in soup plates and pour hot borsch over them.