Preheat oven to 325°F/162°C and line three 7-inch or 8-inch cake pans with parchment paper. DO NOT GREASE THE PANS!! If you accidentally did, wash out the grease and dry them before adding in the parchment paper.
Add 10 egg whites, 2 Tbsp of fresh lemon juice, 1 Tbsp of warm water, 1 1/2 tsp of cream of tartar, and 1 tsp of lemon zest into the bowl of a stand mixer fitted with the whisk attachment. Mix on a low speed for 1 minute to combine the ingredients.
Gradually add in 1/2 cup of granulated sugar. Increase the speed to medium-high and mix until soft peaks have formed and the mixture has quadrupled in volume.
Sift 1 cup cake flour, 1 1/4 cups powdered sugar, and 1 tsp salt into the egg mixture. Use a rubber spatula to gently fold the dry ingredients into the whipped egg whites in 2 additions. Mix just until you can't see any streaks of flour (it's ok if there are some small lumps).
Divide the batter evenly between the prepared pans and smooth out the batter with the back of your rubber spatula. Bake for 22-25 minutes or until a toothpick comes out clean and the tops are lightly browned. Do NOT open the oven door while the layers bake, or they can deflate!
Place the cake layers upside down on a metal cooling rack and let the cake layers cool fully in the pan. Don't worry, the layers won't fall out of the pan! Once they're fully cooled, run an offset spatula around the edge of the pan to carefully loosen the layers.
Remove the cake layers from the pans, then set them aside while you make the whipped cream frosting.