Stir together flour, sugar, orange zest, baking powder, salt, and 1/2 cup of the cocoa in a large bowl. Stir together milk, orange juice, butter, and eggs in a separate bowl. Add milk mixture to flour mixture; stir until smooth and well combined. Add chopped chocolate to the batter; stir to combine, and spread into prepared baking dish.
Add brown sugar and remaining 1/4 cup cocoa to boiling water; stir to combine. Pour boiling mixture over batter in baking dish. Bake in preheated oven until fluffy, crisp, and cooked through (except the sauce at the base of the dish). Serve immediately with your preferred topping of ice cream, yogurt, or cream.