Line a 9x13 baking pan with parchment paper and preheat the oven to 340°F.
Beat the softened butter and sugar together on high speed for 1 minute or until light and fluffy.
Add the first egg and mix until fully incorporated, then add the second.
In a separate bowl, whisk the flour, cocoa powder, and baking powder together until evenly combined.
Gradually add the dry ingredient mixture to the butter mixture, alternating with the heavy cream. Mix just until smooth after each addition. Do not overmix.
Pour the batter into the prepared baking pan and spread it evenly with a spatula.
Bake for 15 to 20 minutes or until a toothpick inserted in the center comes out clean. Let the brownie base cool completely before adding the topping.
Beat the cream with the whisk attachment of an electric mixer until soft peaks form. You can also add powdered sugar and vanilla extract if you want, then continue beating until stiff peaks form. Spread it immediately across the entire surface of the cooled brownie using an offset spatula.
Arrange blueberries in the top left corner of the frosted brownie. Pack them closely into a rectangle to form the canton.
Arrange the sliced strawberries in horizontal rows for the flag's red stripes.
Pipe more whipped cream in each gap between the strawberry rows for the flag's white stripes and you're cake is done.