Preheat the oven to 350 degrees F. Place the rack in the lower third of your oven. Make sure all bowls and utensils are very clean and grease free. Combine ½ cup of the sugar with the flour and sift together four times. Set aside.
Place the 1 ½ cups egg whites in a very clean, grease free stainless steel bowl and fit an electric mixer with a whisk attachment. Beat the whites at medium speed until foamy. Add the salt and cream of tartar, and beat on high speed until the whites form soft peaks. With the mixer on medium speed, gradually add the remaining 1 cup of sugar. Continue to beat on high for a few minutes until the whites are thick and shiny, and glossy peaks form.
Add the vanilla and beat until incorporated. Remove the bowl from the mixer and fold in the lemon zest. Sift about ¼ of the flour mixture over the egg whites and fold in with a rubber spatula. Continue adding the flour mixture by fourths by sifting and folding until it is all incorporated. Spoon the batter into an ungreased 10-inch tube pan with removable bottom, smooth the top, and bake for 35 to 45 minutes, or until it well browned and springs back to the touch.
Remove the cake from the oven and invert the pan on a bottle or metal funnel. Cool completely. Loosen cake from sides and bottom of pan using a metal cake spatula or knife. Invert the cake onto a serving plate. Slice and serve.
The sugar in this cake can be increased from 1 ½ cups to up to 2 cups if you like a sweeter cake. This cake may be stored in an airtight container at room temperature for up to two days.