Line the rectangular baking pan with parchment paper and preheat the oven to 340°F.
Cream the butter and sugar on high speed for 1 minute or until light and fluffy.
Add the eggs, one at a time. Mix after each addition.
In a separate bowl, whisk together the flour, cocoa powder, and baking powder.
Add the dry ingredients to the wet ingredients, alternating with the heavy cream. Mix until smooth.
Pour the batter into the prepared baking pan, spreading it out evenly.
Bake for 15 to 20 minutes or until a toothpick inserted in the center comes out clean. Let the brownie cake cool completely.
Spread a layer of whipped cream over the cake. You can use an offset spatula for a smooth layer.
Arrange blueberries in the top left corner for the stars of the American flag.
Slice the strawberries and line them in rows to form the flag's stripes.
Pipe the remaining whipped cream between the strawberry rows to finish the red and white stripes.