Step 1: Prepare the Cake Batter

1. Preheat and Prepare Pans: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, use a hand mixer or stand mixer to cream the softened butter, oil, and sugar together until light and fluffy, about 2-3 minutes.
4. Add Eggs and Vanilla: Beat in the eggs, one at a time, followed by the vanilla extract. Mix until smooth.
5. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the butter mixture in three additions, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined; do not overmix.
6. Add Vinegar and Color: Stir in the vinegar, then add the purple food coloring a little at a time until the desired color is achieved.

Step 2: Bake the Cakes

1. Divide Batter: Evenly divide the batter between the prepared cake pans and smooth the tops with a spatula.
2. Bake: Bake for 28-32 minutes, or until a toothpick inserted into the center comes out clean.
3. Cool: Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Step 3: Make the Cream Cheese Frosting

1. Cream Butter and Cream Cheese: In a large bowl, beat the softened cream cheese and butter together until smooth and creamy.
2. Add Sugar and Vanilla: Gradually add the powdered sugar, one cup at a time, beating until fully incorporated. Mix in the vanilla extract and a pinch of salt.

Step 4: Assemble the Cake

1. Trim Layers: If needed, trim the tops of the cake layers to make them level.
2. Frost the Cake: Place one cake layer on a serving plate or cake stand. Spread an even layer of frosting on top. Place the second layer on top and frost the top and sides of the cake.
3. Decorate: Garnish with purple sprinkles, edible glitter, fresh berries, or edible flowers as desired.