Preheat your oven to 175°C (350°F). Grease your loaf pan and line it with parchment paper for easy removal.
In a large bowl, beat the softened butter and sugar together with an electric mixer until the mixture is light, fluffy, and pale in color, about 3-4 minutes. This creates a creamy base for your batter.
Add the eggs one at a time, beating well after each addition to ensure they are fully incorporated. The batter should become smooth and glossy, with a slight sheen. Scrape down the sides of the bowl as needed.
In a separate bowl, whisk together the sifted flour, baking powder, and a pinch of salt. Gradually add the dry ingredients to the wet mixture, alternating with the milk, beginning and ending with the dry ingredients. Mix just until combined, being careful not to overmix.
Divide the batter evenly into two bowls. To one, add the cocoa powder and gently fold until fully incorporated, creating a rich chocolate batter. Keep the other as vanilla.
Spoon alternating dollops of vanilla and chocolate batter into the prepared loaf pan. Be generous and aim for a layered effect, but don’t smooth it out yet.
Use a toothpick or skewer to gently swirl through the batter, creating a marbled pattern. Drag lightly from top to bottom and side to side, but avoid overmixing to keep the swirl distinct.
Bake in the preheated oven for about 55 to 65 minutes. Check for doneness by inserting a toothpick into the center; if it comes out clean, the cake is ready. The top should be golden and slightly cracked.
Allow the cake to cool in the pan for about 15 minutes. Then, transfer it to a wire rack and let it cool completely before slicing to reveal the beautiful marbled pattern.
Slice the cake with a serrated knife to showcase the swirl. Serve as is or dust lightly with powdered sugar for an elegant finish. Enjoy the moist, dense texture with its stunning visual appeal!
