Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Cream butter and sugar until smooth and fluffy.
Add in the egg. Beat until combined.
Mix in the baking powder and salt. Gradually fold in flour until a soft dough forms.
Press and cut the dough into rectangles and arrange them on the prepared baking sheet.
Bake for 12 to 15 minutes or until the edges are lightly golden. Cool completely before decorating.
Whip the heavy cream until fluffy. Spread an even layer of whipped cream over the cookie base.
Place 9 blueberries in the upper-left corner of the rectangle.
Add strawberry slices for the flag's red stripes.
Finally, pipe whipped cream between the red stripes for the flag's white stripes.