Heat the oven to 350°F (175°C). Grease a 9 × 5-inch metal loaf pan and line it with parchment, leaving an overhang. Whisk flour, baking powder and salt in a bowl.

Beat softened butter and sugar for about 3 minutes until lighter. Beat in the eggs one at a time, then vanilla. On low speed, add the flour mixture in three additions alternating with 150 ml milk in two additions. Stop as soon as the flour disappears; scrape the bowl and fold briefly.

Whisk cocoa with the remaining 30 ml milk into a paste. Transfer about one-third of the batter to a second bowl and fold in the paste. Keep the two batters separate.

Alternate spoonfuls of vanilla and cocoa batter in the pan. Draw a skewer through the batter in two or three broad passes. Avoid repeated stirring, which blends away the contrast. Smooth the top gently.

Bake for 50–60 minutes. Check the center with a skewer: it should emerge without wet batter, though a few moist crumbs are fine. If the top browns early, cover loosely with foil and continue baking. Oven and pan differences affect the timing.

Rest in the pan for 15 minutes, lift out with the parchment and cool on a rack for about 1 hour before slicing into 10 pieces. Do not taste raw batter; wash hands and utensils that touched raw flour or egg.