Warm up your oven to 350°F (175°C) and apply a thin layer of grease to a 13x9-inch pan.
Whisk the oil, vanilla, sour cream, heavy cream, and egg whites together in a large bowl until smooth. Add the cake mix and stir just until combined.
Fill the pan with batter and bake for about 25 minutes, checking with a toothpick to make sure it comes out clean.
Allow the cake to cool briefly, then poke evenly spaced holes throughout using a fork, about every half inch.
Mix the strawberry Jello with boiling water, followed by adding the cold water. Spread this over the cake, letting it seep into the holes.
Chill in the refrigerator for at least 5 hours to allow it to set.
To make the frosting, whisk together the pudding mix and cold milk until smooth, then gently fold in the whipped topping and a dash of salt. Spread evenly over the chilled cake.
Arrange sliced strawberries and blueberries in a flag design, then refrigerate until it’s time to serve.