Preheat the oven to 325ºF. Grease two 9-inch cake pans with shortening, then dust with flour. You can also line them with parchment paper and spray with nonstick cooking spray. Set the greased pans aside.
In a large mixing bowl whisk together, cake mix, all-purpose flour, and granulated sugar.
Slowly add in water, sour cream, eggs, egg white, heavy cream and vegetable oil, continuing to mix while you are adding.
Scrape the sides of the bowl and continue to mix all until well combined.
Divide cake batter between two prepared 9-inch cake pans.
Bake in preheated oven for 25 minutes or until the center bounces back when you touch it.
Remove from the oven and allow to cool for 15 minutes on a cooling rack.
While the cake is cooling, prepare the Jello. In a mixing bowl add 1 cup of boiling water to the strawberry Jello until whisk until dissolved. Repeat in another bowl with the blue Jello.
Use a fork or a bamboo stick to poke holes all over the cake.
Pour strawberry Jell-O carefully over one of the baked cakes. Carefully, pour berry blue Jello over the other cake.
Cover the cakes and refrigerate for 4 hours or overnight.
Make the frosting:
With an electric mixer, cream together butter and shortening until smooth and creamy, about 3 minutes, scraping the sides of the bowl as needed.
Add in heavy whipping cream and vanilla extract. Mix well.
Slowly add in confectioner’s sugar a 1/2 cup at a time. If frosting is too thick you can add 1 teaspoon of heavy whipping cream until you have reached the texture you are looking for.
Mix frosting for another 2 minutes on medium speed.
Use gel food coloring to color some of the frosting if you'd like red or blue swirls. This is optional.
Assemble the cakes:
Remove cakes from cake pans. Place 1 cake layer on a cake stand and spread about 1 cup of frosting on top.
Place remaining cake on top of frosting. Spread the top and the sides with a light coating of frosting until the cake is completely covered. This is called a crumb coat and will "set" the cake for its final frosting.
Refrigerate the cake for 30 minutes.
Spread another layer of white buttercream
Decorate the cake with colored frosting (use a large piping tip: Wilton 1M or 2D) and sprinkles.
Place cake in the refrigerator until ready to serve.
Refer to the article above for more tips and tricks.The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. The frosting makes the calories higher, so to lower the calories, use less frosting. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**