Preheat your oven to 325 degrees F. Spray three 8-inch round pans with nonstick spray, line the bottom with parchment paper and spray again. Set aside.
In a medium bowl, whisk together flour, salt, and cocoa powder. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, combine the butter and sugar. Beat on medium speed until light and fluffy, about 2 minutes. Scrape down the sides of the bowl and mix again for another minute.
Add the eggs, one at a time, being sure to mix and scrape down the sides of the bowl between each addition. Once all of the eggs are added, continue to beat on medium-high speed for about two mintues, until the mixture gets lighter in color and bigger in volume.
With the mixer on low speed, add 1/3 of the flour mixture, followed by 1 cup of the buttermilk. Continue with another 1/3 of the flour, the second half of the buttermilk and then the remaining flour mixture. Stir until combined.
In a small bowl, combine the vinegar and baking soda. The mixture will bubble. Add the mixture to the cake batter and stir on low until combined. Scrape down the sides of the bowl and mix again for another 20 seconds.
Fold in the sour cream until no streaks remain.
Divide the batter evenly between the prepared pans (21 ounces in each of the 8-inch pans). Bake for about 35-40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs on it. Allow to cool in pans for 15 minutes before inverting onto wire racks to cool completely.