Pre-heat oven to 325°
Whisk together flour, cornstarch and ¼ cup of the sugar. Reserve ½ cup sugar for the next step.
In a clean bowl of a mixer with the whisk attachment, beat the egg whites on medium speed with the cream of tartar (or lemon juice or distilled vinegar if using instead), vanilla, and salt until foamy. About 1 minute.
Increase the speed to high and gradually add in the remaining 1/2 cup sugar, 1 tablespoon at a time. Beat until soft hilly peaks form about 3-4 minutes. Do not beat into stiff peaks.
Use a rubber spatula to gently fold in the flour ingredients. Add about 1/4 at a time to the egg whites. You want to incorporate all of the flour without deflating the egg whites. Take your time and gently fold. You may have a few small lumps, just make sure there is no dry flour left.
Gently pour/spoon the batter into the loaf pan and evenly spread. Bake at 325° for 35-40 minutes or until golden brown and no longer sticky on top.
Turn the pan upside down and rest on top of two jars or cans. Cake must cool upside down for at least 1 hour. Angel food cake will deflate if it cools right-side up.