Preheat your oven to 350°F (175°C) and prepare a 9x5-inch loaf pan by greasing it and dusting it with flour.

In a large mixing bowl, combine the dry ingredients: flour, sugar, cocoa powder, baking powder, and salt. Whisk together.

In a separate bowl, cream the softened butter and sugar mixture until light and fluffy, about 2-3 minutes.

Add the eggs and vanilla to the creamed mixture and beat until well combined.

Gradually add the dry ingredients to the wet mixture, along with the milk, orange zest, and orange juice. Mix until just combined.

Divide the batter into two bowls. Stir cocoa powder into one bowl to create the chocolate batter.

Spoon alternating dollops of the orange and chocolate batters into the prepared loaf pan and swirl gently with a knife.

Bake in the preheated oven for 50-60 minutes, checking for doneness with a toothpick.

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