1. Preheat your oven to 375°F and prepare an ungreased 9×13 cake pan.
2. Sift half of the sugar with the cake flour and salt five times; set aside.
3. In a large mixing bowl, whip the egg whites on low speed until frothy. Add the vanilla extract and cream of tartar.
4. Gradually increase to medium speed, adding the remaining sugar slowly until stiff, glossy peaks form.
5. Carefully fold in the flour mixture. Pour the batter into the prepared pan, smooth the top, and release air bubbles by running a knife through the batter.
6. Bake for 15-20 minutes, or until the cake is golden and a toothpick comes out clean. Cool the cake upside down for one hour, supported by cans.
7. To prepare the whipped cream, beat together the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
8. For the classic flag design, spread the whipped cream over the cooled cake. Arrange blueberries in the top left corner and sliced strawberries in rows to resemble the red and white stripes of a flag. Use additional whipped cream to create white stripes.
9. For the trifle option, prepare the pudding with milk according to package instructions. Layer chopped pieces of angel food cake, strawberries, pudding, blueberries, and whipped cream in a trifle dish. Repeat the layers and decorate with berry patterns. Chill the trifle for 2-3 hours before serving.