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.

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 powdered sugar and mix until well combined.

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

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

Arrange the sliced strawberries in the shape of stripes and place blueberries in the top left corner to create the flag design.

If desired, brush the strawberries with corn syrup for a glossy finish.