Preheat the oven to 350 degrees. Spray 2 (8-inch) round cake pans with cooking spray. (Be sure your pans are at least 2 inches tall, because the cake layers rise quite a bit.) Line the bottom of the pans with parchment paper, then spray the paper with cooking spray.

Combine the cake mix, flour, sugar, sour cream, water, vegetable oil, egg whites, and vanilla extract in a large mixing bowl.

Beat with an electric mixer on medium speed for 2 minutes—the batter should be very smooth. Use a rubber spatula to scrape up from the bottom and get everything totally mixed together.

Divide cake batter between the prepared pans and bake cake layers 43 to 48 minutes, until a cake tester inserted into the center of the cakes comes out clean.

Cool cake layers in the pans for 20 minutes, then carefully transfer to a wire rack to cool completely.

Once the cake layers are fully cooled, use a long serrated knife to trim them if they’ve domed.

Use a fork with large tines to poke holes in the cake every half inch or so; try not to go all the way to the bottom of the cake. (If you don’t have a large-tined fork, you can use a toothpick but you'll want to swirl the toothpick around a bit to enlarge some of the holes.)

Combine the jello and boiling water in a small bowl and stir until the jello dissolves. Set aside to cool for 5 minutes.

Pour half of the jello over each cake layer. (To avoid a mess on your counters, set the cakes on a wire rack and set the wire rack over a large baking sheet. That should contain any spilled jello.)

Refrigerate the cake layers for at least 4 hours. You can also refrigerate them overnight, but you should cover them loosely with plastic wrap if you do.

Place the butter in a large mixing bowl and beat until very smooth.

Add 4 1/2 cups of the powdered sugar, 5 tablespoons of the milk, and the vanilla and salt. Beat on low to combine.

Turn mixer to medium and beat until frosting is very smooth.

If frosting is too thin, add additional powdered sugar; it it’s too thick add additional milk.

Set one chilled cake layer on a cake stand or cardboard cake circle. Spread a thick layer of frosting over the cake and top with the second layer.

Frost with the remaining frosting and decorate as desired.

(I reserved some of the frosting, dyed it green, and piped it around the edge of the cake. See the body of this post for details if you want.)

Serve. Uneaten cake can be stored loosely covered in the fridge for up to 3 days or in the freezer (tightly wrapped) for up to 3 months.