Preheat the oven to 325°F (160°C).
Sift together half of the sugar and cake flour.
In a large mixing bowl, beat the egg whites until foamy on a low speed. Add cream of tartar and salt and continue to beat, gradually increasing the speed to medium high.
When the egg whites turn white with denser foam, stir in remaining ¾ cup of sugar one tablespoon at a time. Continue to beat on medium high speed until soft peaks form.
Stir in orange zest and pure vanilla extract.
If needed, transfer the meringue into a larger mixing bowl.
Add flour/sugar mixture into the meringue and carefully fold the batter until all the dry ingredients are mixed in. Be careful not to deflate the batter.
Now, transfer the batter into ungreased angel food cake pan. Smooth out the surface and run a butter knife through the batter to remove any air pockets. Bake the cake for 45-55 minutes, or until inserted wooden stick comes out clean.
Cool the cake completely in the pan upside down, about 2 hours. I prefer to balance the pan upside down on an inverted mug as pictured above. This will make sure the cake stays light and fluffy while cooling.