Preheat the oven to 350 degrees and grease a loaf pan.
Beat the butter and sugar together in a large bowl until light. Add the eggs one at a time, mixing well after each addition. Stir in the milk and vanilla extract.
In a separate bowl, whisk the flour, baking powder, and salt. Add the dry mix to the wet ingredients and stir until just combined.
Scoop half the batter into another bowl and stir the cocoa powder into it. Alternate spoonfuls of the plain and chocolate batters in the loaf pan. Swirl gently with a knife to create a marble effect.
Bake for 45 to 50 minutes until a toothpick comes out clean. Cool in the pan for 10 minutes, then remove to a rack. Once cool, slice the loaf into thick pieces so the marble pattern shows like bones.