Pre heat oven to 350 F. Spray donut pan with spray oil, and rub to distribute oil thoroughly.
In a small bowl, combine flour, ½ cup sugar and lemon zest. Set aside.
In a stand mixer fitted with a whisk, beat egg whites on medium high for around 2 minutes, until they are frothy and have loose peaks (will stand for a second and then sink back into the eggs).
Add a pinch of salt and cream of tartar, continue to beat on medium-high for 3-5 more minutes until stiff peaks form (won't sink back into eggs).
At this point, continue beating on medium speed and gradually add the ⅓ cup portion of granulated sugar *by the tablespoon* over a period of 3-5 more minutes. Stop and scrape the sugar/eggs down from the bowl every few minutes.
Once sugar has dissolved completely (check by running meringue between your fingers, if no grains remain, you are done), fold in the vanilla extract followed by the flour/sugar/lemon zest mixture (best to do this in several batches, and *be gentle*)
Spoon mixture into donut pans, filling right to the top.
Bake at 350°F for 11-13 minutes, until tops become lightly golden brown.
Allow to cool for 5 minutes, before gently loosening edges with a knife. Invert pan over a wire rack and give the donut pan a firm smack to get the donuts out.