1. Preheat oven to 350°F (175°C). Grease, line with parchment, and flour three 8-inch round cake pans.2. In a large bowl, whisk together the dry ingredients: flour, sugar, baking soda, salt, and cocoa powder.3. In a separate medium bowl, whisk together the wet ingredients: vegetable oil, eggs, buttermilk, purple food coloring, vanilla extract, and white vinegar.4. Pour the wet ingredients into the dry ingredients. Mix on low speed with an electric mixer until just combined.5. Divide the batter evenly between the three prepared pans (use a kitchen scale for precision).6. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.7. Let the cakes cool in the pans for 15 minutes, then invert them onto a wire rack to cool completely.8. To make the frosting: In a large bowl, beat the softened cream cheese and butter on medium-high speed until smooth (3-4 minutes).9. Reduce speed to low, gradually add the powdered sugar, then add vanilla and salt.10. Increase speed to medium-high and beat for 3 minutes until light and fluffy.11. To assemble: Level the cooled cake layers with a serrated knife. Place one layer on a cake stand. Top with 1 cup of frosting.12. Add the second layer, top with 1 cup of frosting. Add the third layer, upside-down for a flat top.13. Apply a thin “crumb coat” of frosting over the entire cake and chill for 30 minutes.14. Apply the final, generous layer of frosting to the top and sides of the cake.