Preheat the oven to 350°F (175°C). Grease and flour a loaf pan.

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In another bowl, combine the flour, baking powder, and salt. Gradually add to the creamed mixture alternately with milk.

In a separate bowl, mix the cocoa powder with boiling water until smooth.

Divide the batter in half. Stir the cocoa mixture into one half of the batter.

Spoon the vanilla and chocolate batters alternately into the prepared loaf pan.

Use a knife to gently swirl the batters together to create a marbled effect.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.