Preheat the oven to 350°F and spray a 12x17x1-inch half sheet pan with non-stick spray.

Sift together the flour, baking powder, and salt in a bowl.

In a stand mixer, cream the sugar and butter until fluffy.

Add the eggs one at a time, then stir in the vanilla extract.

Mix in the sour cream and vegetable oil until well combined.

Gradually incorporate the flour mixture and buttermilk, mixing until everything is just blended.

Spread the batter evenly in the prepared pan and bake for 16-18 minutes. Allow the cake to cool completely.

For the frosting, mix the cold milk and powdered sugar until dissolved, then add the pudding mix and beat until it thickens.

Gently fold in the thawed whipped topping until light and fluffy.

Frost the cooled cake with two-thirds of the frosting, leaving some for decoration.

Decorate the cake with blueberries and strawberries to represent the American flag, piping the remaining frosting in between.