Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan.

In a mixing bowl, cream together the unsalted butter and granulated sugar until light and fluffy, about 3 minutes.

Add the eggs, one at a time, beating well after each addition.

Stir in the milk until well combined.

In another bowl, whisk together the all-purpose flour and baking powder.

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

Divide the batter in half. In one half, stir in the cocoa powder.

Spoon alternating dollops of vanilla and chocolate batter into the prepared loaf pan. Use a knife to gently swirl them together for a marbled effect.

Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before serving.