Sift the flour, baking soda, and salt in a medium bowl.
Beat the eggs and add them to the flour.
Add the sugar, yogurt(or Kefir), and vanilla and mix well.
Heat the oil on medium-high, then reduce the heat to medium-low. Make sure the oil temperature is about 365-375F.
Drop the batter by the tablespoon, 4-5 at a time, into the hot oil and fry for a minute or until golden. Flip the doughnuts and continue to fry them until done.
Drain them on paper towels.
Dust the doughnuts with powder sugar and serve warm.
Tip: Fry only one donut at the beginning and ensure it is cooked inside. Reduce the heat if necessary, so the dough cooks through.