Preheat oven to 350°F (180°C). Spray 3 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.
In the large bowl of a stand mixer, whisk together flour, sugar, baking powder, salt, and baking soda by hand. Add butter; using the paddle attachment, beat at medium-low speed until butter is incorporated and mixture resembles coarse crumbs, about 2 minutes.
In a medium bowl, whisk together milk, egg whites, sour cream, and vanilla. With mixer on low speed, add one-third of milk mixture (about 2/3 cup or 180 grams) to flour mixture. Increase mixer speed to medium, and beat until light and fluffy, 1 to 2 minutes. Gradually add remaining milk mixture, beating until well combined and stopping to scrape sides of bowl. Divide batter among prepared pans (about 1 1/2 cups or 356 grams each).
Bake until a wooden pick inserted in center comes out clean, 20 to 30 minutes. Let cool in pans on wire racks for 10 minutes. Remove from pans, and let cool completely on wire racks.
In another medium bowl, stir together 3 cups (690 grams) Vanilla-Almond Buttercream, cherries, and pecans.
Level cooled cake layers, if desired. Place 1 cake layer on a serving plate. Spread half of cherry buttercream mixture on top. Top with second cake layer, and spread remaining cherry buttercream mixture on top. Top with remaining cake layer.
In a small bowl, reserve 1 cup (212 grams) Vanilla-Almond Buttercream. Spread remaining buttercream on top and sides of cake. Using an offset spatula, scrape excess frosting off sides of cake to smooth and expose cake layers, if desired. Refrigerate assembled cake for at least 15 minutes.
Transfer reserved buttercream to a pastry bag fitted with a medium open star piping tip (Ateco #826). Pip buttercream on top of cake as desired. Cover and refrigerate for up to 3 days.