1. Preheat oven to 375°F and prepare an ungreased 9×13-inch cake pan.
2. Sift half the sugar with cake flour and salt five times; set aside.
3. Whip egg whites until frothy; add vanilla and cream of tartar.
4. Gradually add remaining sugar to egg whites while whipping until stiff, glossy peaks form.
5. Gently fold in flour mixture 1/4 cup at a time.
6. Pour batter into the pan, smooth the top, and run a knife through to remove air bubbles.
7. Bake for 15-20 minutes until golden and a toothpick comes out clean.
8. Cool the cake upside down for about an hour using cans or similar objects.
9. Loosen cake edges and remove from pan once cooled.
10. Whip the cream with powdered sugar and vanilla until stiff peaks form. Reserve 1 1/2 cups for piping.
11. Spread the rest of the whipped cream evenly over the cake.
12. Create the flag design: arrange blueberries in the top left for stars, lay sliced strawberries in rows for stripes, and pipe the reserved whipped cream to create white stripes.
13. Refrigerate until serving.