Preheat your oven to 175°C (350°F). Grease your loaf pan and line it with parchment paper for easy removal.
Using a mixer or whisk, cream the softened butter and sugar together on medium speed until the mixture is pale, fluffy, and fragrant—about 3-4 minutes. You’ll notice it lightens in color and feels smooth and slightly thickened.
Add the eggs one at a time, beating well after each addition. The batter should become smooth and glossy, with a slightly thickened texture as the eggs incorporate fully.
Stir in the vanilla extract, letting the aroma fill your kitchen as the mixture turns fragrant and creamy.
In a separate bowl, sift together the flour and baking powder to ensure an even, lump-free mixture. Gradually add this dry mixture to the wet batter on low speed, alternating with the milk. Mix just until the ingredients are combined—don’t overmix, or the batter could become dense.
Divide the batter evenly into two bowls. To one, gently fold in the cocoa powder until smooth, creating a rich chocolate batter.
Pour the vanilla batter into your prepared loaf pan, spreading it evenly. Then, dollop the chocolate batter on top in spoonfuls.
Using a skewer or toothpick, gently swirl the chocolate and vanilla layers together. Make slow, figure-eight motions to create a beautiful marbled pattern—avoid overmixing to keep the distinct swirls intact.
Bake for approximately 55-60 minutes, until the top is golden and a toothpick inserted into the center comes out moist with crumbs. If the crust browns too quickly, loosely cover with foil around the 45-minute mark.
Remove the cake from the oven and let it cool in the pan for 15 minutes. Then, transfer it to a wire rack and cool completely—about 1 hour—until the crumb feels set and the pattern is clear.
Slice the cooled cake with a serrated knife, revealing the marbled pattern inside. Serve at room temperature and enjoy the moist, tender crumb with its stunning visual contrast.
