In the bowl of a stand up mixer, place the yeast, one teaspoon of the sugar and add about 1/4 cup of lukewarm milk.
Once the yeast has activated and bloomed, add the remaining sugar, the eggs (beaten) and half the flour.
Knead with the dough hook while adding the liquids, and the zest.
ATTENTION: add the flour a little bit at a time, you might not need all of it.
Add the flour until the dough is smooth but sticky, knead for about 8 minutes.
Now add the salt and let go for 2 more minutes.
Remove the dough from the bowl, place it in an oiled container and let it rise for about 2 hours, it should double in size.
When the dough has doubled in volume, transfer it to a flat working surface and roll it out to about 1/2 inch thickness.
Using a 3 or 3 1/2 inches donuts cutter, cut circles close to one another to minimize waste.
Re-knead the dough and cut again, letting the dough rest for about 10 minutes in between.
Place the donuts on the floured board and cover with a clean towel until almost doubled in size.
Pre-heat the air fryer to 325°.
When ready spray with vegetable oil and place the risen donuts upside down (the flat side on the bottom of the board should be now on top)
Cook for about 4 minutes, rotate the donuts to ensure even browning and cook 2 to 5 minutes more, depending on the size of the air fryer and donuts.
Let cool on a cooling rack. Repeat with all the remaining donuts.
