Preheat the oven to 350°F and line a baking sheet with parchment paper.
Beat the softened butter and sugar together on medium-high speed for about 2 minutes or until smooth and fluffy.
Add the egg and beat until fully incorporated, about 30 seconds.
Mix in the baking powder and salt, then gradually fold in the flour until a soft dough forms. Do not overmix.
Turn the dough out onto the prepared baking sheet and divide it into equal portions. Press each piece into a rectangle, leaving at least 1 inch between the rectangles on the baking sheet.
Bake for 12 to 15 minutes or until the edges are lightly golden and the centers look set. Let the sugar cookies sit on the pan for 5 minutes, then transfer to a wire rack. Cool completely before decorating.
Whip the heavy cream until fluffy. Spread an even layer of whipped cream over each cookie.
Add the blueberries to the top left corner and arrange them into a tight rectangle for the flag's canton.
Lay the strawberry slices in horizontal rows for the flag's red stripes.
Finally, whip the remaining heavy cream into stiff peaks and transfer to a piping bag. Snip a small corner and pipe it into the gaps between the strawberry rows for the flag's white stripes.