Preheat the oven to 350 degrees F. Combine 1/2 cup of sugar with the flour and sift together. Set aside.
Place the egg whites, salt, and cream of tartar in the bowl of an electric mixer fitted with a whisk attachment and beat on high speed until the eggs make medium-firm peaks, about 1 minute.
With the mixer on medium speed, add the remaining 1 1/2 cups of sugar by sprinkling it over the beaten egg whites. Whisk for a few minutes until thick and shiny.
Whisk in the vanilla and lemon zest and continue to whisk until very thick, about 1 more minute. Sift about 1/4 of the flour mixture over the egg whites and fold it into the batter with a rubber spatula. Continue adding the flour by fourths by sifting and folding until it’s all incorporated.
Pour the batter into an ungreased 10-inch tube pan, smooth the top, and bake it for 35 to 40 minutes, until it springs back to the touch and a toothpick comes out clean. Remove the cake from the oven and invert the pan on a cooling rack until cool.