Step 1: Prep the Cake Pans

1. Preheat your oven to 350°F (175°C).

2. Grease two 9-inch round cake pans with butter.

3. Line the bottoms with parchment paper, grease again, and lightly dust with cocoa powder (instead of flour to avoid white residue on the cake).

Step 2: Mix the Dry Ingredients

1. In a medium bowl, sift together:

   • Flour

   • Cocoa powder

   • Baking powder

   • Baking soda

   • Salt

2. Whisk to combine and set aside.

Step 3: Cream the Butter and Sugar

1. In a stand mixer (or large bowl with a hand mixer), beat butter on medium speed for 3–5 minutes until soft and fluffy.

2. Add sugar and continue beating for another 1–2 minutes until light and airy.

Step 4: Incorporate Eggs and Vanilla

1. Add eggs, one at a time, beating well after each addition (about 1 minute per egg).

2. Scrape down the sides of the bowl, add vanilla extract, and beat for an additional minute.

Step 5: Mix the Wet Ingredients

1. In a 2-cup measuring cup, combine:

   • Buttermilk

   • White vinegar

   • Purple food coloring (see notes for shade adjustments).

2. Stir gently to mix.

Step 6: Combine Wet and Dry Ingredients

1. Alternate adding the dry ingredients (in 3 parts) and the buttermilk mixture (in 2 parts) to the butter mixture.

2. Begin and end with the dry ingredients, mixing just until combined (do not overmix).

3. Scrape the bowl and ensure the batter is evenly mixed.

Step 7: Bake the Cake

1. Divide batter evenly between the prepared cake pans.

2. Smooth the tops and tap each pan gently on the counter to remove air bubbles.

3. Bake on the middle oven rack for 25–30 minutes, or until a toothpick inserted in the center comes out clean.

4. Let cakes cool in the pans for 10 minutes, then turn out onto wire racks to cool completely.

Step 8: Make the White Chocolate Cream Cheese Frosting

1. Melt white chocolate in a double boiler over low heat, stirring occasionally.

2. Once melted, remove from heat and let it cool slightly.

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

4. Add vanilla extract and cooled white chocolate, beating until fully incorporated.

5. If using food coloring, mix in drops until you reach the desired purple shade.

Step 9: Assemble the Cake

1. Place one cake layer on a serving plate and spread a layer of frosting on top.

2. Add the second cake layer and apply a thin crumb coat of frosting.

3. Chill for 15–30 minutes, then apply a final layer of frosting.

4. Smooth the frosting with an offset spatula for a polished look.