Heat the oven to 350°F (175°C). Lightly grease a 9-by-5-inch loaf pan with a little butter from the measured amount and line it with parchment, leaving overhang on the long sides.

Whisk the flour, baking powder and salt in a bowl. In a small separate bowl, stir the cocoa and hot water into a smooth paste; allow it to cool slightly.

Beat the remaining softened butter and sugar with a hand mixer for 2–3 minutes until pale and fluffy. Beat in the eggs one at a time, then the vanilla. Add the dry mixture in two additions, alternating with the milk, and mix on low only until no dry flour remains.

Transfer about one-third of the batter to the emptied dry-ingredient bowl. Fold in the cocoa paste just until the color is even.

Spoon the vanilla and chocolate batters alternately into the prepared pan. Draw a butter knife through the batter in 2–3 gentle, broad passes; avoid mixing the colors completely.

Bake for 50–60 minutes, checking at 50 minutes, until the center springs back lightly and a skewer inserted into the center has no wet batter. If the top darkens too quickly, tent loosely with foil. Cool in the pan for 15 minutes, then lift out and cool on a wire rack for about 1 hour before cutting into 10 slices.