Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, cream together the softened butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, combine the flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, alternating with the milk. Mix until just combined.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool completely in the pan on a wire rack.
Once cooled, spread the whipped cream evenly over the top of the cake.
Arrange the sliced strawberries in a rectangle in the top left corner to create the 'flag' design. Use the blueberries to fill in the remaining area of the flag.
Chill the cake in the refrigerator for at least 30 minutes before serving to set the whipped cream.