Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, using an electric mixer, beat the butter and sugar on medium-high speed until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.

Reduce the mixer speed to low and add the flour mixture in three additions, alternating with the milk, beginning and ending with the flour mixture. Mix until just combined.

Pour the batter into the prepared pan and spread it evenly.

Bake for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Allow the cake to cool completely in the pan on a wire rack.

In a large bowl, using an electric mixer, beat the butter on medium speed until creamy, about 2 minutes.

Gradually add the powdered sugar, one cup at a time, beating on low speed until well combined.

Add the heavy cream, vanilla extract, and salt. Increase the mixer speed to high and beat for 3 minutes, until the frosting is light and fluffy.

If the frosting is too thick, add more heavy cream, one tablespoon at a time, until you reach the desired consistency.

Once the cake is completely cool, spread a layer of the frosting evenly over the top of the cake.

In the upper left corner of the cake, arrange the blueberries in a rectangular shape to represent the stars of the American flag.

Place the sliced strawberries in rows across the cake to represent the stripes of the flag. Alternate rows of strawberries with rows of plain white frosting to mimic the flag's design.

Chill the cake in the refrigerator for at least an hour before serving to allow the frosting to set and the flavors to meld.

Cut into squares and serve.