Preheat oven to 175 °C (350 °F).
Cream butter and sugar in a bowl until light and fluffy.
Add the egg and beat until fully combined.
Stir in baking powder and salt, then gradually mix in flour until the dough comes together.
Press the dough evenly (about ¼–⅓-inch thick) onto a parchment‑lined baking tray (preferably rectangle).
Bake 12–15 minutes until edges are lightly golden. Remove and let cool completely.
Evenly spread whipped cream over the cooled base.
In the top-left corner, place blueberries in a compact cluster to form the “stars” field.
Arrange strawberry slices horizontally across the rest of the base to create red stripes.
Pipe or spoon clean lines of whipped cream between the strawberry stripes to complete the white stripes.
Chill briefly if needed before slicing into flag‑shaped pieces.