For the cake: Preheat oven to 325°F. Prepare 2 9-inch cake rounds with pan spray and flour.
Sift the flour twice – once before you measure and then again after you measure. Sift together the baking soda, sugar, and cocoa. Mix with the flour in a large mixing bowl.
In a medium bowl, whisk together buttermilk, vegetable oil, eggs, vanilla extract, food coloring, and vinegar.
In the large bowl, make a deep well in the middle of the dry ingredients and pour in all the wet ingredients (this is very important). Using an electric mixer, mix on medium speed until all ingredients are well blended – approximately 3 minutes. The batter will be very thin.
Divide the batter evenly into the 2 cake rounds. Once the batter is in the pans, gently drop the pans on the counter to remove any air bubbles.
Bake until a toothpick inserted in the center comes out clean – approximately 30-45 minutes (cake should be very moist so be careful not to overbake).
Cool cake in pans on rack for 10 minutes. Loosen around edges with a small knife. Remove cake from pans and cool completely on rack to room temperature.
For the frosting: Using an electric mixer on medium speed, blend the butter, cream cheese, and powdered sugar together until smooth – approximately 3 minutes. Add the pecans to the mixture and gently mix until fully blended.
Place cake layers on a cake plate, spreading frosting between each layer. Spread remaining frosting on top and side of cake.