Preheat oven to 350°F and grease a 13” x 9” baking pan with nonstick cooking spray. In a large bowl, sift flour, baking powder and salt together until combined. In another large bowl, using a hand mixer, beat sweetener and oil spread on medium speed until light and fluffy. Add eggs and vanilla and beat until well combined.
Add dry ingredients and milk to wet ingredients and beat until just combined. Pour batter evenly into prepared pan. Bake until a toothpick inserted into the center comes out clean, about 30-40 minutes. Poke cake with the end of a wooden spoon in 1-inch intervals. Allow to cool in pan completely before frosting, about 30 minutes.
Spread pudding evenly over cake, gently pushing it into the holes. Cover and refrigerate until pudding is set, 1-2 hours.
Spread whipped topping in an even layer over cake. Place strawberries and blueberries on top of cake in the shape of an American flag. Slice and serve.
To make the pudding:
In a large bowl, whisk milk and pudding mix together until combined and slightly thickened.