Preheat oven to 325 degrees. DO NOT grease or line loaf pan.
In a medium bowl, whisk 1/4 cup of sugar, flour and cornstarch. Set aside
In a mixing bowl of a stand mixer with a whisk attachment, beat the egg whites, lemon juice, vanilla and salt until foamy, about 30 seconds.
Increase mixing speed to high and with mixer running, gradually add in the remaining 1/2 cup sugar, a bit at a time. Beat until soft peaks form, 2-4 minutes. Do not beat into stiff peaks.
Remove mixing bowl from mixer and gently fold in the flour ingredients, about a 1/4 cup at a time. Incorporate flour without deflating the egg whites.
Transfer batter to loaf pan and bake 30-40 minutes until brown and the top no longer feels sticky.
When baked, remove the pan from oven and turn upside down and rest the handles on top of two jars or canned goods. Let cool upside down for one hour.
Once ready to serve, run a butter knife along the edges. Lay pan on its side and remove from pan. Cut into slices with a serrated knife and serve.