Preheat the oven to 325°F (165°C)
Sift cake flour and sugar together
Separate egg whites from yolks
Add cream of tartar and salt to the egg whites
Beat egg whites until foamy
Add sugar gradually while beating
Beat until stiff, glossy peaks form
Fold in vanilla extract
Gently fold in the sifted flour mixture in small batches
Pour batter into an ungreased angel food cake pan
Smooth the top lightly
Bake until golden and springy
Invert the pan immediately after baking
Let the cake cool completely upside down
Remove the cake from the pan
Slice with a serrated knife