Lightly butter two 8-inch round cake pans, then dust with a little flour or line with parchment paper for easy removal. In a medium bowl, sift together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.

Using a mixer, beat the softened butter and granulated sugar until light and fluffy, about 3-4 minutes. Add eggs one at a time to the butter-sugar mixture, beating well after each addition. Mix in vanilla extract.

Blend in the purple gel food coloring and white vinegar until the batter turns a vibrant purple. Gradually add the flour mixture and buttermilk in alternating batches to the batter, beginning and ending with the dry ingredients. Mix until just combined avoid over mixing.

Evenly divide the batter between the prepared pans. Smooth the tops and bake in a preheated oven at 350°F (175°C) for 30 to 35 minutes, or until a toothpick inserted comes out clean.

Remove the cakes from the oven and let them cool in the pans for 10 minutes. Then transfer to a wire rack to cool completely before frosting.

Gently melt the white chocolate using a double boiler or microwave, stirring often to prevent burning. In a mixing bowl, beat the softened butter and cream cheese together until smooth and creamy.

Slowly add the melted white chocolate and vanilla extract, continuing to mix until the frosting is well blended and fluffy.

If you want a hint of color, stir in a small drop of purple gel food coloring to tint the frosting. Place one cake layer on your serving plate and spread an even layer of frosting over it. Top with the second cake layer.

Use the remaining frosting to cover the entire cake, smoothing the top and sides evenly. Chill the assembled cake in the refrigerator for about 30 minutes to let the frosting set before slicing and serving.