1. Preheat the oven to 350°F. Grease and flour a 17-by-12 1/2-inch sheet cake pan. If desired, halve the recipe for a smaller pan.
2. In a mixer, beat the butter and sugar on high speed until light and fluffy. Add the eggs two at a time, mixing well after each addition. Stir in the sour cream and vanilla extract.
3. In a separate bowl, sift together the flour, cornstarch, salt, and baking soda. Gradually add dry ingredients to the butter mixture on low speed until just combined.
4. Spread the batter evenly in the prepared pan. Bake for 20-30 minutes. Check doneness with a toothpick at 20 minutes; it should come out clean.
5. For the frosting, beat cream cheese and butter until smooth. Add powdered sugar and vanilla extract, mixing until well combined.
6. Let the cake cool completely before applying the frosting.
7. Decorate the cooled cake by spreading frosting evenly. Use a pastry bag with a star tip to pipe frosting for decoration.
8. Mark off a square in the top left corner of the cake for the star area using a toothpick. Fill this area with blueberries.
9. Alternate piped lines of frosting and rows of raspberries horizontally across the cake to create the stripes of the flag.