1. Preheat your oven to 375°F and prepare an ungreased 9×13 inch cake pan.
2. Sift half the sugar with the flour and salt multiple times, then set aside.
3. In a large bowl, whip the egg whites on low speed until frothy; add the cream of tartar and vanilla extract.
4. Gradually add the remaining sugar to the egg whites while whipping until stiff, glossy peaks form.
5. Carefully fold in the flour mixture and pour the batter into the cake pan, smoothing the top.
6. Bake for 15-20 minutes until golden, then let the cake cool upside down.
7. To prepare the whipped cream, mix the cream, powdered sugar, and vanilla until stiff peaks form.
8. Spread the whipped cream over the cooled cake, reserving some for decoration.
9. Arrange blueberries in the top left corner for the stars and strawberries in stripes for the flag.
10. Pipe the reserved whipped cream between the strawberry rows for white stripes.
11. Refrigerate until serving, adding fresh fruit just before serving if preparing ahead.