Preheat your oven to 175°C (350°F). Grease a 9x5 inch loaf pan and line it with parchment paper for easy removal.
In a large mixing bowl, cream together the softened butter and superfine sugar until the mixture is light, fluffy, and fragrant—about 3-4 minutes on medium speed. You'll notice it turns pale and feels smooth to the touch.
Add eggs one at a time, beating well after each addition. The batter should become glossy and slightly thickened, with a gentle ribboning when you lift the beaters.
Mix in the vanilla extract until fully incorporated. The aroma will deepen and the batter will look smooth and uniform.
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 in three parts, alternating with the milk, starting and ending with the flour. Mix just until combined—don’t overmix, or the cake may become dense.
Divide the batter evenly into two bowls. Sift the cocoa powder into one bowl and gently fold it in until the batter is uniformly dark, creating a rich chocolate color.
Spoon dollops of vanilla and cocoa batter into the prepared loaf pan, alternating between the two. Use a skewer or knife to gently swirl the batters together—just a few gentle strokes to create a marbled pattern, without fully mixing the colors.
Bake in the preheated oven for 55-60 minutes. Check for doneness by inserting a toothpick into the center; it should come out with moist crumbs, not wet batter. The crust should be golden and crackly.
Allow the cake to cool in the pan for about 15 minutes. Then, transfer it to a wire rack and let it cool completely—this helps the crumb set and prevents crumbling when slicing.
Slice the cake with a serrated knife to reveal the beautiful marbled pattern inside. Serve slices slightly warm or at room temperature, enjoying the tender crumb and rich aroma of vanilla and cocoa.
