Preheat oven to 190º C.
Grease a tray for 12 muffins.
In a large bowl, mix together dry ingredients - sugar, orange zest, flour, baking powder, salt, and chocolate.
In a medium bowl, lightly beat wet ingredients - eggs and melted butter, sour cream, milk, orange juice, and vanilla extract.
Make a well in the dry ingredients and pour wet ingredients into it, then stir just enough to combine. Do not over mix; at most 20 stirs.
Divide the batter amongst the muffin cups.
Bake for 15-20 minutes (a toothpick should come out clean).
Leave for 10 minutes in the tray, remove muffins from tray, and leave to cool on a wire rack.