Make the cake:

1. Preheat the oven to 350°F . Grease a 9 x 13-inch baking pan and line with parchment paper.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.  Set aside.
3. In a large mixing bowl, use an electric mixer to cream the butter and sugar together until light and fluffy (about 3–4 minutes).
4. Beat in the eggs one at a time, then mix in the vanilla.
5. Add the flour mixture to the wet ingredients in three additions, alternating with the milk. Mix until just combined.
6. Pour batter into the prepared pan and smooth the top. Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean.
7. Let the cake cool completely in the pan.

Make the frosting

1. Use an electric mixer to beat cream cheese and butter until smooth and fluffy (2-3 minutes).
2. Gradually add powdered sugar, beating until combined.
3. Add vanilla and beat until smooth. Add the milk 1 tablespoon at a time until the frosting reaches your desired consistency.

Assemble and decorate.

1. Once fully cooled, cut the cake in half crosswise to create two 9×6.5-inch rectangles.  Place each half on its own platter or serving board.  You can either decorate both cake halves at once, or wrap one half in plastic wrap and save it for later!
2. Spread a generous layer of frosting over the cake(s).
3. Create a square in the upper left corner with blueberries.
4. Use strawberry quarters to make red stripes across the rest of the cake, leaving frosting visible between rows for white stripes.
5. Optional step: If you have leftover frosting , add it to a piping bag with a star tip. Pipe it in between the rows of strawberries to bring your flag design to the next level!