Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. In a bowl, whisk together flour, cocoa powder, salt, and baking soda. In a separate large bowl, mix the sugar, oil, eggs, buttermilk, vanilla, and vinegar. Add the dry ingredients gradually, mixing until just combined. Stir in the purple food coloring until the batter is a deep, vibrant shade.