Step 1: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan, line the bottom with parchment paper, and grease the parchment paper. Then, flour the pan, tapping out any excess.
Step 2: In a large bowl, cream together the softened unsalted butter and granulated sugar with an electric mixer until light and fluffy, about 3-5 minutes.
Step 3: Beat in the large eggs, one at a time, ensuring each is fully incorporated before adding the next. Scrape down the sides of the bowl as needed.
Step 4: Stir in the pure vanilla extract.
Step 5: In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.
Step 7: Pour the batter evenly into your prepared 9×13-inch baking pan. Spread gently with a spatula.
Step 8: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. The top should be golden brown and spring back when lightly touched.
Step 9: Allow the cake to cool in the pan on a wire rack for 15-20 minutes before inverting it onto the wire rack to cool completely. Ensure it is completely cool before frosting.
Step 10: To make the frosting, in a large bowl, beat the softened cream cheese and unsalted butter together with an electric mixer until smooth and creamy.
Step 11: Gradually add the sifted confectioners’ sugar, mixing on low speed until combined, then increase to medium speed and beat until fluffy, about 2-3 minutes.
Step 12: Stir in the pure vanilla extract. Taste and adjust sweetness if desired.
Step 13: Once the cake is completely cool, spread the cream cheese frosting evenly over the top of the cake using an offset spatula.
Step 14: Arrange the fresh blueberries in a square pattern in the upper left-hand corner of the frosted cake to represent the stars of the flag.
Step 15: Arrange the fresh strawberries in alternating rows to create the red stripes, leaving space for the white stripes (the frosting itself). You can slice the strawberries or halve them for this.
Step 16: Continue arranging the strawberries to fill the remaining ‘red stripe’ areas, creating neat and distinct lines.
Step 17: For optimal flavor and presentation, chill the assembled cake for at least 30 minutes before serving. Store any leftover cake in an airtight container in the refrigerator for up to 3-4 days.