Pre heat oven to 350 F.
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 muffin pan lined with muffin papers. The batter will stick out of the top of the muffin papers but it won't overflow while baking. Bake at 350 F for 15-18 minutes, until tops become golden brown. Set aside to cool.