Preheat the oven to 350°F. Butter and flour three 6-inch cake pans.
In a medium bowl, sift together the flour, salt, and baking soda.
In a large mixing bowl or using a stand mixer, cream the butter until smooth. Add sugar and beat until light and fluffy.
Beat in eggs, one at a time, then add vanilla extract.
Sift cocoa powder into the butter mixture and beat until combined. Scrape down the bowl.
In a liquid measuring cup, mix buttermilk, vinegar, and red food coloring.
With the mixer on low speed, alternate adding flour mixture and buttermilk mixture to the batter.
Divide batter among the prepared pans and bake for 30-35 minutes, or until the centers spring back when touched.
Let cakes cool in pans for 10 minutes before transferring to a wire rack to cool completely.