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

In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.

In another bowl, combine the milk, melted butter, eggs, and vanilla extract. Mix well.

Gradually add the wet ingredients to the dry ingredients, mixing 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.

Allow the cake to cool completely in the pan on a wire rack.

In a mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar and mix until smooth.

Add the milk and vanilla extract, mixing until the frosting is fluffy.

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

Arrange the blueberries in the top left corner to form the stars and the stripes with the sliced strawberries and raspberries.

Slice and serve the cake chilled or at room temperature.