Preheat the oven to 350°F.

Line a muffin tin with paper liners or spray with non-stick cooking spray.

In a large bowl, whisk together the flour, baking powder, baking soda, salt, and orange zest.

In a smaller bowl, whisk together the brown sugar, egg, oil, milk, orange juice, and vanilla.

Pour the wet mixture into the dry ingredients and mix until just combined.

Move half of the batter to another bowl and set aside.

In a small, microwave safe bowl, heat the chopped chocolate in the microwave at 50% power, stirring every 30 seconds until it is almost melted. Continue stirring until it is completely smooth.

Add the melted chocolate to one half of the muffin batter, and stir to combine.

To the other bowl of batter, fold in the mini chocolate chips.

Fill the muffin cups 2/3 full with alternating spoonfuls of the two batters.

When all the batter is used up, use a toothpick to swirl the batter in each muffin tin.

Bake for 15-18 minutes, or until a toothpick inserted into the centre of a muffin comes out clean.

Let the muffins cool in the pan for 5-10 minutes, and then move them carefully to a wire rack to cool completely.

In a small bowl, whisk together the powdered sugar and orange juice to make the glaze.

Spoon to the glaze over top of the cooled muffins and let it set.