Preheat oven to 350℉ (180℃). Note: do not grease angel food cake pan.
Make lemon sugar: Put the sugar in a bowl and add the lemon zest. Use your fingertips to rub the zest into the sugar for about one minute.
¾ Cup Granulated Sugar (measure #2), 2 Tablespoons Lemon Zest
Place cake flour, lemon sugar and salt in a medium-sized bowl. Whisk to combine and distribute ingredients evenly. Set aside for later.
1 Cup Cake Flour, 1 teaspoon Salt
Place the egg whites, warm water, lemon juice, and cream of tartar into the bowl of a stand mixer.
1 ½ Cups Egg Whites, ⅓ Cup Warm Water, 2 Tablespoons Lemon Juice, 1 teaspoons Cream of Tartar
Using the whisk attachment, begin mixing on medium speed until egg whites become foamy.
While the mixer is running, slowly add the sugar to the bowl a little at a time. This should take about 2 to 3 minutes.
1 Cup Granulated Sugar (measure #1)
Increase mixer speed to medium-high and continue whipping until stiff, glossy peaks are reached. (Note: Stiff peaks are when egg whites hold their shape firmly and don’t collapse. They have a shiny surface and a slightly elastic texture)
Remove bowl from the stand mixer.
Add the combined dry ingredients to the stiffened egg whites in three additions, folding gently between each addition. Take care to not overfold else you will deflate the whipped eggs.
Pour cake batter into an ungreased 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.