Beat 1-1/3 cups of the flour and remaining ingredients (except oil and cinnamon/sugar mixture)with an electric mixer on low speed, until just blended.
Stir in remaining flour and beat on medium speed for an additional 2 minutes.
Cover dough and refrigerate for about 1 hour (dough should be stiff after refrigeration).
Heat 2 to 3 inches of vegetable oil in a fryer or heavy (3 quart) saucepan to 375°F (for perfect doughnuts use a themometer).
Divide dough in half and roll each half out on a well floured pastry board to 3/8″ thick. Cut with a well floured doughnut cutter, or use a 3″ cup and a clean pop bottle lid for the center.
Carefully slide cut doughnuts into hot oil, using a wide spatula. Cook 2 or 3 doughnuts at a time, so as not to lower the tempeture of the oil too much. Fry 1 to 1-1/2 minutes on each side or until golden brown.
Carefully remove from the oil (do not prick) and drain on papertowels.
If desired, sprinkle with cinnamon/sugar mixture while still hot.