1. Preheat oven to 350 degrees. Coat cake pans with cooking spray; cover bottoms with parchment, and coat again; set aside. Sift together flour, baking powder, salt, and baking soda in a large bowl; set aside.
2. Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until fluffy, about 12 minutes. Mix in vanilla. Add eggs, 1 at a time, mixing well after each addition. Add flour mixture in 2 batches, alternating with the buttermilk. Divide batter among prepared pans. Gently toss berries in flour to coat completely, and gently press into batter.
3. Bake until a cake tester inserted in centers comes out clean. (If the cakes become too dark while baking, cover with foil.) Transfer to wire racks; let cool in pans slightly. Invert cakes onto racks. Remove parchment, and reinvert. Cakes can be wrapped in plastic and stored up to 1 day.
4. To assemble: Review Cake Assembly instructions. Cut each cake in half horizontally, making 4 layers of each size. Spread buttercream on 3 cake layers (see note below for amounts). Stack layers, placing the plain layer on top. Coat tiers with buttercream; chill. Smooth fondant over tiers.