Preheat the oven to 325 degrees F. Spray the bottom and sides of three 8-inch or four 6-inch round cake pans with nonstick spray, line the bottom with parchment paper, and spray again with the nonstick spray. Set aside.
In a stand mixer fitted with the paddle attachment, combine the butter, granulated sugar, brown sugar, and red velvet flavor. Beat the ingredients on medium-high speed for about 5 minutes, until the mixture is light in color and slightly increased in volume.
Lower the speed and add the eggs, one at a time, mixing until each egg is incorporated before adding the next. Continue to beat on medium-high until smooth and nearly double in volume.
In a separate bowl, whisk together the flour and cocoa powder. Sprinkle the salt on top of the dry ingredients and whisk until combined.
With the stand mixer on low speed, alternately add the dry ingredients and the buttermilk into the butter mixture, beginning and ending with dry ingredients. Mix until just incorporated. Fold in the sour cream.
In a small bowl, combine the vinegar and baking soda until it fizzes and then add to the batter. Stir until blended.
Divide the batter evenly between the prepared pans (about 16 oz. of batter in each 8-inch or 12oz in each 6-inch).
Bake for 25 to 30 minutes, or until a toothpick inserted in the center of the cake comes out with a few moist crumbs on it.
Let pans cool on wire rack for about 10-15 minutes before inverting the cakes onto racks. Let cool completely.
Level cakes if needed. Chill the cakes for at least an hour before using. Can be stored the freezer for up to one month wrapped well in plastic wrap and foil.