Preheat the oven to 350 degrees F. Spray a 9-inch by 5-inch loaf pan with non-stick cooking spray, line with parchment paper and then spray the paper.

In a medium bowl, combine the flour, baking powder and salt. In a separate bowl, whisk together the melted butter and cocoa powder until smooth.

In a large bowl, use an electric mixer to beat the room temperature butter and sugar until smooth. Beat in the eggs, vanilla and milk until fully combined. Fold in the dry ingredients. Stir 1 cup of the batter into the cocoa powder mixture.

Spread a third of the vanilla batter into the prepared loaf pan. Alternate spoonfuls of chocolate and vanilla batter. Tap the pan on the counter to make the batter settle evenly. Use a knife to swirl and marble.

Bake for 25 minutes. Lower the oven temperature to 325 degrees F and bake for another 40-45 minutes until the cake is golden brown and a toothpick inserted into the center comes out clean. If the cake starts to brown too quickly, cover loosely with foil while baking.

Cool the cake in the pan for 10 minutes before removing and cooling completely on a wire rack.