Preheat oven to 350°. Note: do not grease angel food cake pan.
Make lemon sugar: Place sugar measure #2 (¾ cup) in a bowl and add lemon zest. Work the lemon zest into the sugar by rubbing together with your fingertips for one minute.
Place cake flour, lemon sugar and salt in a bowl. Whisk to combine.
Add egg whites, warm water, lemon juice and cream of tartar in the bowl of a stand mixer.
Using the whisk attachment, begin mixing on medium speed until egg whites become foamy.
Slowly start adding sugar measure #1 (1 cup) into mixing bowl while the mixer is running. This process will take 2 - 3 minutes.
Increase mixer speed to medium-high and continue whipping until stiff, glossy peaks are reached.
Remove bowl from the stand mixer.
Add the combined dry ingredients to the stiffened egg whites in three additions, folding gently between each addition.
Pour cake batter into angel food cake pan. Carefully smooth the top with a rubber spatula.
Bake for 30 minutes or until well risen and a toothpick inserted in the center comes out clean.
Remove from oven and immediately invert the cake pan over onto a cooling rack.
Let cake cool upside down for 1 hour (leave pan on cake).
Gently lift pan off cake. If cake is sticking to pan, run a sharp knife around the inside of the pan to release.