Preheat your oven to 350°F and line a 9x13-inch baking pan with parchment paper. Cream together the butter and sugar for about 2 minutes until it's pale and fluffy. This step matters because it adds air to your batter, making the cake lighter and more tender. Don't rush it.

Beat in the eggs one at a time, making sure each one is fully incorporated before adding the next. Add the vanilla extract and mix for another 30 seconds. The mixture should look smooth and creamy at this point.

In a separate bowl, whisk together the flour, baking powder, and salt. Alternate adding the dry ingredients and milk to your butter mixture, starting and ending with the dry ingredients. Mix until just combined—don't overmix or your american flag sheet cake will turn out tough and dense.

Pour the batter into your prepared pan and spread it evenly using an offset spatula or the back of a spoon. Bake for 28-32 minutes until a toothpick inserted in the center comes out clean. The top should be golden brown and spring back when you lightly touch it.

Let the cake cool completely in the pan for at least 30 minutes. I always let mine cool for a full hour because it's much easier to arrange the fruit when the cake is completely set. Patience here prevents your berries from sinking into warm cake.

Dust the entire cake lightly with powdered sugar once it's cooled. This creates a beautiful backdrop for your fruit and adds a touch of sweetness that complements the berries perfectly.

Arrange your fresh fruit in the flag pattern: a blue rectangle of blueberries in the upper left corner for the canton, then red and white stripes across the cake using strawberries and raspberries alternating with the powdered sugar. Step back and admire your patriotic creation before slicing.