1Preheat your oven to 175°C (350°F). Grease and flour a loaf pan or line it with parchment paper.
In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
In a large bowl, beat the eggs and granulated sugar together until light and fluffy.
Add the sour cream, vegetable oil, and vanilla extract to the egg mixture. Mix until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Divide the batter evenly into two bowls. In one bowl, sift in the unsweetened cocoa powder and mix until fully incorporated.
Alternate spoonfuls of the plain and chocolate batters into the prepared loaf pan. Use a knife or skewer to gently swirl the batters together to create a marbled effect.
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing and serving.