Prepare Your Pans: Preheat oven to 350°F (175°C). Grease two 9-inch round cake pans, line with parchment paper, then grease the parchment and dust with cocoa powder.

Mix Dry Ingredients: Sift together cake flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl. Set aside.

Cream Butter and Sugar: In a stand mixer, cream butter on medium-high speed until light (about 3-5 minutes). Gradually add sugar, beating until fluffy.

Add Eggs and Vanilla: Beat in eggs one at a time, ensuring each is incorporated before adding the next. Add vanilla extract and mix well.

Combine Dry and Wet Ingredients: Stir buttermilk with vinegar and purple food coloring in a jug. Alternate adding the dry ingredients and buttermilk mixture to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.

Bake: Divide batter between prepared pans. Bake for 25 minutes, or until a toothpick inserted in the center comes out clean. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Prepare the Frosting: Melt white chocolate in a double boiler and set aside to cool slightly. In a large bowl, beat cream cheese and butter until creamy. Add melted white chocolate and vanilla extract, mixing until smooth. Add food coloring if desired.

Assemble the Cake: Spread frosting on one cooled cake layer, top with the second layer, and cover the entire cake with remaining frosting.