Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large bowl, sift together the flour, sugar, baking soda, salt, and cocoa powder.
In another bowl, whisk together the vegetable oil, buttermilk, eggs, white vinegar, and vanilla extract.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Add the purple gel food coloring, mixing until the desired shade is achieved.
Divide the batter evenly between the prepared cake pans.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
For the frosting, beat the softened cream cheese and butter together until smooth and creamy.
Gradually add the powdered sugar and vanilla extract, beating until light and fluffy.
Once the cakes are completely cool, frost the top of one cake layer, then place the second layer on top.
Frost the top and sides of the entire cake with the cream cheese frosting.
Decorate as desired and serve.