For the Cake

1. Preheat the oven to 350°F and set an oven rack in the middle position. Butter and flour a 9x13-in metal pan, or use nonstick cooking spray with flour in it, such as Baker's Joy or Pam with Flour.
2. In a medium bowl, whisk together the flour, salt and baking soda. Set aside.
3. In the bowl of an electric mixer fitted with the paddle attachment (or beaters), beat the butter and sugar on medium speed until light and fluffy, about 5 minutes. Beat in the vanilla, then beat in the eggs, one at a time, scraping down the sides of the bowl as necessary. (Don't worry if the batter looks a little curdled at this point.)
4. Reduce the speed to low, then beat in one-third of the flour mixture, followed by half of the buttermilk. Scrape down the sides and bottom of the bowl, then add another third of the flour, followed by the remaining buttermilk. Beat in the remaining flour, then scrape down the bowl and beat again until the batter is evenly combined. Do not overmix.
5. Scrape the batter into the prepared baking pan and bake for 30 to 35 minutes, until the cake is set and lightly golden around the edges, and a toothpick inserted into the center comes out clean. Set the cake on a wire rack to cool completely.

For the Frosting and Topping

1. In the bowl of a stand mixer fitted with the paddle attachment (or large bowl if using a hand mixer), combine the cream cheese, butter, vanilla and salt. Mix on low speed until combined, then increase the speed to medium-high and beat until aerated and light, about 2 minutes. Gradually add 4 cups of the confectioners' sugar, mixing on low speed to combine. Once the confectioners' sugar is mixed in, increase the speed to medium-high and beat until fluffy, about 1 minute. Set aside 1 tablespoon of the frosting (you'll need it for the blueberry "stars.")
2. When the cake is completely cool, use an offset spatula to spread the frosting evenly over top. The frosting won't show so no need to fuss over making it perfect.
3. Outline the "stars" section on the upper left corner of the cake with a toothpick (or skip this step and just eyeball it) and fill with a generous layer of blueberries. Dip a small handful of the remaining blueberries in the reserved frosting, and then in the remaining 1½ cups of confectioners' sugar to make white "stars." Evenly scatter the "stars" over the section of blueberries. Arrange 1 row of raspberries across the top of the cake, laying them sideways, like a red stripe. Gently roll the next row of raspberries in the confectioners' sugar to make a white stripe, and alternate until the flag is completed. (I suggest using one hand for rolling and one for placing; if any powdered sugar accidentally gets on the red "stripes," use your fingertip to dab the berries lightly with water and it will come right off. )
4. Cut the cake into squares and serve directly from the pan.
5. Note: If you’d like to make your own buttermilk, check out the easy method here.
6. Note: The berries should be dry before decorating the cake. (In general, I don't rinse raspberries because they are so delicate.)
7. Make-Ahead Instructions: The cake itself can be made one day ahead of time. Cover with plastic wrap and store at room temperature. Frost and decorate the cake before serving.