Preheat oven to 350°F (175°C). In medium bowl, cream butter and sugar until fluffy. Add egg, salt, cream of tartar, and baking soda, mixing until smooth. Gradually fold in flour until dough forms. Divide dough into 12 equal portions. Shape each into discs, chill for 15 minutes. Place on parchment-lined baking sheets, flatten slightly with palm, then cut 5 notches into each for star shapes. Bake 12-15 minutes until golden. Let cool completely.
In large bowl, beat cream cheese and sifted powdered sugar until smooth. In separate bowl, whip cold whipping cream until stiff peaks form. Gently fold 1/3 cream into cheese mixture, then carefully transfer to a piping bag. Pipe frosting onto cooled cookies. Arrange berries in patriotic color patterns, dust with additional powdered sugar if desired.