CAKE:

• Preheat oven to 350℉. Butter and flour an 18x13 baking sheet; set aside.
• In the bowl of a stand mixer fitted with a paddle attachment, cream the butter and sugar until light and fluffy. On medium speed, add in the eggs one at a time, mixing just until combined. Add the sour cream and vanilla and mix just until combined. Be sure to scrape the sides often during this process.
  1 cup (226g) unsalted butter, room temperature, 2 cups (420g) granulated sugar, 5 large eggs, room temperature, 1 cup (250g) sour cream, room temperature, 2 teaspoon vanilla
  
• In a large bowl, whisk together the flour, cornstarch, baking powder, and salt. Add to the butter mixture and mix on LOW. Mix just until combined. Pour into prepared pan and smooth with a spatula. Bake for 18-22 minutes or until a toothpick comes out clean. Cool completely before frosting.
  2¾ cups (370g) all-purpose flour, ⅓ cup (50g) cornstarch, 2½ teaspoons baking powder, ½ teaspoon salt
  
• To make the frosting, beat the butter and cream cheese in the bowl of a stand mixer fitted with a paddle attachment until smooth. Add the powdered sugar and vanilla and beat until smooth.
  1½ cups (339g) unsalted butter, room temperature, 3 (8 ounce) (678g) packages cream cheese, room temperature, 4 cups (480g) confectioners sugar, 2 teaspoons vanilla
  
• Spread ½ of the frosting across the cake. Arrange the fruit to resemble the American flag. With the remaining frosting, pipe the white stripes and stars if desired.