Preheat oven to 300°F/150°C. Butter an 8.5 x 4.5-inch or a 9×5-inch loaf pan.
In a medium bowl whisk together flour, baking powder, and salt. Set aside.
In a mixer bowl fitted with the paddle attachment, 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 vanilla and 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. Avoid mixing to the point where all the batter turns chocolaty.
Bake for 80-90 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.
Cake will keep for up to 5 days in the refrigerator. Alternatively, it can be frozen for 2 months. To thaw, leave on the counter, still covered, or overnight in the fridge.