Heat the oven to 350 °F, then butter and flour three 6-inch cake pans. Coat the pans evenly, especially around the bottom edge, so the layers release cleanly.
Sift the flour, salt, and baking soda into a medium bowl. The dry mixture should look uniform with no visible clumps of baking soda.
Beat the butter on medium speed, add the sugar, and beat for about 3 minutes before adding the eggs one at a time and then the vanilla. The mixture should become paler, fluffy, and fall from the paddle in a thick ribbon.
Sift the cocoa into the butter mixture and mix only until it disappears, then scrape the bowl. Stop as soon as the batter looks evenly cocoa-tinted.
Stir the buttermilk, vinegar, and red food coloring together in a liquid measuring cup. The liquid should be evenly colored before it reaches the batter.
With the mixer on low, add half the dry mixture and half the buttermilk mixture, then repeat with the remaining portions and divide the batter among the pans. Scrape the bowl as needed and stop mixing when no flour streaks remain.
Bake for about 30 to 35 minutes, cool the layers in their pans for about 10 minutes, then turn them onto a wire rack to cool completely. The centers should spring back lightly when pressed and the layers must be fully cool before frosting.