Preheat oven to 200 degrees C. Lightly grease a 12 cup muffin pan, or line with patty cups.
In a medium bowl, mix flour, dark chocolate, baking powder, bicarb soda and salt.
In a separate medium bowl, cream together margarine and sugar until smooth. Beat in the eggs one at a time then stir in the orange zest, orange juice and buttermilk. Pour into the flour mixture and mix just until evenly moist. Spoon batter into muffin cups.
Bake for 15 to 20 minutes in the preheated oven, or until a toothpick inserted into the centre of a muffin comes out clean. Let muffins cool in the pan on a wire rack for at least 10 minutes before removing from the pan.