Stir the yeast and sugar into the water and let the mixture sit for a few minutes until all the yeast is dissolved.
Combine the salt with 1 cup of flour in a mixing bowl. Make a well in the middle of the flour and add in the yeast and water mixture. Mix the liquid into the flour until it turns into a thin batter. Cover with a cloth and let it sit for 30 minutes.
Using an electric stand mixer, gradually stir in the remaining flour until you get a soft dough. Knead the dough until smooth and elastic.
Cover the dough with a towel again and let it sit for an hour, or until it doubles in size.
Place the dough on a floured surface and stretch it into a long log. Divide the log into 20 even pieces.
With your finger, punch out a hole in the middle of one of the dough pieces. Repeat until all the dough pieces have a hole in the center.
Heat the oil until it begins sizzling over medium-low heat. Drop the donuts in one-by-one into the oil, cooking until golden on each side.
Lift the donuts out with a skewer and place them on a paper towel lined plate. Toss the donuts in sugar when they are still warm.