Preheat your oven to 350F and lightly coat an 8"x4" loaf pan with non-stick spray and line with parchment.
Combine in a medium bowl, the flour, cocoa powder, baking powder, baking soda, salt, and cinnamon. Stir with a whisk to mix and set aside.
1 cup All Purpose Flour, ½ cup Cocoa Powder, 1 teaspoon Baking Powder, ½ teaspoon Baking Soda, ¼ teaspoon Salt, 1 teaspoon Cinnamon
Add to a large mixing bowl, the softened butter and granulated sugar. Cream with a whisk or hand blender, until lighter in color. Crack in the eggs, then add in the vanilla and orange zest. Mix until smooth.
½ cup Unsalted Butter, ¾ cup Granulated Sugar, 2 large Eggs, 1 teaspoon Vanilla, 1 teaspoon Orange Zest
Pour ¼ of the dry ingredients into the wet ingredients and mix until combined. Pour in the orange juice and mix until smooth.
½ cup Orange Juice
Continue alternating between adding some of the flour mixture and the milk to the bowl of batter. Mix until batter comes together.
¼ cup Milk
Scrape the batter into the prepared loaf pan. Level off the batter with a spatula for an even rise.
Bake the load in the preheated oven until a toothpick inserted in the middle comes out clean. Let the loaf cool completely.
Make the icing by placing the icing sugar in a small mixing bowl. Whisk in a little bit of orange juice into the sugar until your desired consistency and thickness of icing has been reached. Pour over the cooled loaf and let it set before slicing.
¾ cup Icing Sugar, 1 tablespoon Orange Juice