Preheat the oven to 170°C. In a mixing bowl, toss sugar and orange zest together, then add flour, baking powder, cinnamon and a pinch of salt and mix to combine. In a separate bowl, whisk eggs with melted butter, then pour in orange juice and milk and mix until fully combined. Add wet ingredients to the bowl with the dry ingredients and stir until just combined, then add chopped milk chocolate and mix gently - do not overmix, the batter should be slightly lumpy.
Spoon out the batter evenly into the muffin cases lined with liner papers. Bake for 15 minutes or until skewer inserted in the middle of a muffin comes out clean.