Get your oven preheated to 350F. Spray a 9 by 5 pan with a cooking spray. Line the base with a parchment paper spray the paper and set it aside
Melt the butter and stir in the cocoa powder and set aside
Whisk together the flour, baking powder, and salt
In a large bowl cream together the Butter and sugar
Add the Eggs one at a time. Creaming together the mixture after each addition until the Egg is totally absorbed by the cream
Add the Flour and milk alternately starting with the flour and ending with the flour until everything is well combined.
Scoop out about a cup of batter and put that in a separate bowl. Add the melted butter and cocoa mixture and stir together until everything is well blended.
Add half of the white batter in a 9 by 5 loaf pan, followed by half of the dark batter (in three large scoops) in a loaf pan; then use a table knife to make a swirl pattern on the cake. Repeat again with the remaining batter.
Place the pan in a 350F preheated oven and bake for 40 to 45 minutes until a toothpick inserted in the middle of the cake comes out clean.
Leave the cake to cool for about 10 minutes before serving.