Preheat your oven to 350°F. This is crucial for a properly baked cake, so don't skip this step! It helps the cake rise perfectly.
Grease a 19x13 cake pan with butter. After greasing, sprinkle a little flour all over the pan, shaking it gently to remove any excess. This will help the cake release easily once it’s baked.
In a large mixing bowl, sift together the flour, baking powder, baking soda, and salt. This ensures that everything is well combined and prevents lumps in your batter.
In the bowl of a stand mixer, or a large mixing bowl, beat the egg whites with 1 cup of sugar until soft peaks form, about 4 minutes. You'll know it’s ready when you can turn the bowl upside down and nothing falls out!
In another mixing bowl with a paddle attachment, beat the butter with the remaining cup of sugar on medium speed until it’s light and fluffy, about 2 minutes. Make sure to scrape down the sides a couple of times to ensure everything is well mixed.
Add in the whole egg and vanilla extract to the butter mixture, mixing until well combined.
Turn the mixer to medium-low and gradually add the flour mixture in three additions, alternating with the buttermilk. Start and end with the flour. Beat until just combined, scraping down the sides as needed.
Gently fold the beaten egg whites into the batter using a spatula. Be careful not to over-mix; you want to maintain those lovely airy peaks!
Pour the batter into the prepared pan and bake for 30 to 35 minutes. You'll know it's done when a toothpick inserted into the center comes out clean. Let it cool for at least 10 minutes, then invert onto a cooling rack and allow it to cool completely.
While the cake cools, make the frosting by combining the softened butter, cream cheese, powdered sugar, and vanilla extract in a large bowl. Use an electric mixer to beat until smooth and creamy.
Once the cake is completely cool, use an offset spatula to ice the top and sides with your cream cheese frosting. Be generous! It’s the best part.
Now comes the fun part! Arrange the fresh blueberries in the top left corner of the cake to create a rectangle. Then, make four rows of “stripes” with the sliced strawberries across the top of the cake. Use the large star tip to pipe frosting between the strawberry stripes.
For added flair, use the small star tip to pipe 3 to 4 rows of stars across the top of the blueberries. This will give your cake that classic flag look and impress your guests!
