In a medium bowl, whisk together flour, baking powder, and salt. This is the flour mixture.
In a stand mixer bowl, beat the butter for about 1 minute. Add sugar and beat again until it becomes light and fluffy. Add the egg and vanilla until everything is combined. Don’t forget to scrape the sides.
Add the flour mixture into the bowl and mix until combined.
Press the dough into the prepared baking sheet. Refrigerate the dough for about 1 hour.
Preheat oven to 350 degrees F.
Bake for 15-18 minutes until the edges are slightly brown.
Cool on a wire rack.
Cream Cheese Frosting
Place all the ingredients in a stand mixer bowl. Mix together for about 5 minutes, scraping the sides of the bowl every 2 minutes. The frosting should be light and fluffy. Spread the frosting on top of the cake and decorate with the blueberries and strawberries to make the flag.