Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt.

In a separate bowl, whisk together eggs, buttermilk, oil, and vanilla extract. Add the purple food coloring and whisk until evenly combined.

Add the wet ingredients to the dry ingredients and mix until just combined. Do not overmix.

In a small bowl, stir together the white vinegar and 0.5 cup of hot water. The mixture will fizz.

Gently fold the vinegar mixture into the cake batter.

Pour the batter evenly into the prepared cake pans.

Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

In a large bowl, beat together the softened butter and cream cheese until smooth and creamy.

Gradually add the powdered sugar, beating until well combined and smooth. Add vanilla extract.

If the frosting is too thick, add milk or cream, 1 tablespoon at a time, until desired consistency is reached.

Once the cakes are completely cool, place one cake layer on a serving plate. Spread a layer of frosting over the top.

Top with the second cake layer and frost the top and sides of the entire cake.

Decorate as desired. Chill until ready to serve.