Preheat your oven to 350°F.
Combine 1/2 cup of sugar with the cake flour.
Sift this mixture together four times and set aside.
Place the egg whites, salt, and cream of tartar in the bowl of an electric mixer.
Beat on high speed until medium-firm peaks form, about 1 minute.
With the mixer on medium speed, add the remaining 1 1/4 cups sugar by sprinkling it over the beaten egg whites.
Continue to whisk until very thick and shiny, about 5-7 minutes.
Add the vanilla extract and lemon zest.
Whisk until incorporated, about 1 more minute.
Sift about 1/4 of the flour mixture over the egg whites.
Fold it gently into the batter, repeating until all the flour mixture is incorporated.
Pour the batter evenly into an ungreased 10-inch tube pan.
Smooth the top with a spatula.
Bake for 35-40 minutes, until the top is lightly browned and a toothpick comes out clean.
Invert the pan on a cooling rack and let the cake cool completely.
Once cooled, remove the cake from the pan and serve.