Preheat the oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper.

In a large mixing bowl, combine flour, sugar, baking powder, and salt.

In another bowl, whisk together milk, melted butter, eggs, and vanilla extract.

Pour the wet ingredients into the dry ingredients and mix until just combined.

Pour the batter into the prepared baking pan and smooth the top.

Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool completely in the pan on a wire rack.

In a mixing bowl, beat the softened butter until creamy.

Gradually add powdered sugar, alternating with milk, and beat until smooth and fluffy.

Mix in vanilla extract.

Once the cake is completely cool, spread the frosting evenly over the top.

Arrange sliced strawberries in the shape of a rectangle in the top left corner to represent the flag's blue field.

Fill the remaining area with blueberries and whipped cream to create the stripes of the flag.

Chill the cake for at least 30 minutes before serving.