1. Preheat oven to 350°F (175°C). Grease, line with parchment, and flour two 9-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. Do not overmix.5. Divide the batter evenly between the two prepared cake pans.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 and creamy (about 3 minutes).9. Reduce the speed to low and gradually add the powdered sugar. Mix until fully combined.10. Add the vanilla extract and salt. Increase speed to medium-high and beat for 2-3 minutes until light and fluffy.11. Once the cake layers are completely cool, frost the cake. Place one layer on a serving plate, top with frosting, add the second layer, and frost the top and sides.