Arrange a rack in center of oven; preheat oven to 350°. Grease 2 (13" x 9") metal baking pans with cooking spray; line bottom of each with parchment.

In a large bowl (or the large bowl of a stand mixer), whisk cake mix and salt. Add egg whites, sour cream, milk, oil, and vanilla. Using an electric mixer on medium speed (or the paddle attachment of the stand mixer), beat until combined and no lumps remain, about 2 minutes.

Divide batter between prepared pans, smoothing to distribute across bottom of pans (batter will be thicker than typical box cake mix batter).

Bake cakes until lightly browned on top, pulling away from the sides of the pan, and springy to the touch, 20 to 25 minutes.

Let cakes cool at least 10 minutes in pans, then invert onto a wire rack and let cool completely.

Remove parchment. Using a serrated knife, trim domed tops off cakes. Discard cake (or save for snacking!).

In the large clean bowl of stand mixer fitted with the paddle attachment, beat cream cheese, confectioners’ sugar, and salt, scraping down sides of bowl as needed, until smooth, 1 to 2 minutes. Add milk and instant pudding and beat until combined, about 1 minute more. Scrape down bowl, then add heavy cream and continue to beat until thick and light, 1 to 2 minutes more.

Arrange one layer of cake on a platter. Top with mousse. Using an offset spatula, spread in an even layer, leaving a ½" border on all sides. Top with remaining cake, trimmed side down. Refrigerate cake until ready to use.

In the large bowl of stand mixer fitted with the paddle attachment, beat shortening, water, vanilla, and salt on medium-high speed until smooth and creamy.

Add 1 cup (113 g.) confectioners’ sugar and beat on low speed just until combined. Continue to add confectioners’ sugar in 1-cup increments, beating after each addition, until all sugar is added. Increase mixer speed to medium-high and beat until smooth and fluffy, about 1 minute more.

Transfer one-quarter of frosting to a piping bag fitted with a star tip. Using offset spatula, frost top and sides of cake with remaining frosting. Pipe a shell border around top and bottom of cake as desired. Decorate with sprinkles (if using).