Preheat oven to 325°F/160°C. Butter a loaf pan (both 9×5-inch or 8.5x4.5-inch would work) and line it with a piece of parchment paper for easy removal.

In an electric mixer bowl fitted with the paddle attachment or using a handheld mixer, beat butter and sugar on medium speed for 4-5 minutes until light and fluffy. Beat in eggs, one at a time, until combined. Scrape down the sides and bottom of the bowl as necessary. Beat in vanilla extract. On low speed, beat in the flour mixture in 3 additions, alternating with the buttermilk/sour cream in 2 additions, starting and ending with the flour. Do not over mix.

Divide batter in half, preferably with a food scale but you can also eyeball it. Using a rubber spatula, fold the melted white chocolate into one half, and melted dark chocolate and cocoa powder into the other half.

Dollop the white chocolate and dark chocolate mixtures alternately into the pan like a chess board – first next to each other, then in layers on top of each other. When done, insert a knife all the way down and swirl the batter gently, but don't overdo it so as not to combine the batters.

Bake for about 90 minutes or until a toothpick inserted into the center of the cake comes out clean. Let cake cool slightly in pan, then turn out from the pan and allow to cool completely on a wire rack (or, you can leave it to cool in the pan if you prefer). Serve cake at room temperature.