Preheat oven to 350°F.
In a bowl, whisk flour, cornstarch, baking powder, baking soda, and salt.
In a separate bowl, cream butter and sugar until fluffy. Add egg, vanilla, and almond extract.
Mix in dry ingredients just until combined.
Press dough onto a greased or parchment-lined sheet pan in a rectangle. Bake for 15–18 minutes until just golden. Let cool completely.
For frosting, beat cream cheese, powdered sugar, and vanilla until smooth. Spread a thin layer over cooled cookie.
In a separate bowl, whip cream with sugar until stiff peaks form. Fold remaining frosting into the whipped cream to lighten it.
Pipe white stripes and decorate with sliced strawberries and blueberries to resemble the American flag.
Chill until ready to serve.