A rich, moist pound cake swirled with white and bittersweet chocolate batters.
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.
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.