Preheat your 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 another bowl, beat the softened butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Then, mix in the milk and vanilla extract until smooth.

Gradually add the dry ingredients to the wet mixture, mixing just until combined.

Split the batter in half and stir in the cocoa powder to create the chocolate batter.

Alternately spoon the vanilla and chocolate batters into the prepared loaf pan and swirl with a knife.

Bake for about 40 minutes or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring it to a cooling rack.