Preheat the oven to 350°F (175°C).

Grease and flour a 9x5 inch loaf pan.

In a bowl, whisk together the flour, baking powder, and salt.

In a separate bowl, beat the softened butter and granulated sugar until light and fluffy.

Mix in the eggs one at a time, followed by the vanilla extract.

Gradually add the dry mixture to the wet ingredients, alternating with the milk.

In a small bowl, mix the cocoa powder with a tablespoon of the batter until smooth.

Pour half of the vanilla batter into the prepared pan, followed by the cocoa mixture, and then top with the remaining vanilla batter.

Use a knife to gently swirl them together for that marbled effect.

Bake in the preheated oven for 30 minutes.

Cool in the pan for about 10 minutes before transferring to a cooling rack.