Heat the oven to 190°C. Using a pastry brush and 1 tablespoon of oil, lightly grease a muffin pan with cups measuring 6 cm across.
Purée the bananas with 1 tablespoon of oil, the whole egg and egg whites and the milk.
Sift the flour, sugar and bicarbonate of soda into a large mixing bowl, and stir in the orange zest and chocolate. Fold in the banana mixture until just blended; if overmixed, the muffins will be heavy. It does not matter if a little flour remains unincorporated.
Spoon the mixture into the prepared muffin pan, filling the cups about two-thirds full. Bake for 25 minutes, or until the muffins have risen and are lightly browned.
Remove from the oven and take the muffins out of the pan. Serve them hot or at room temperature, preferably on the day of baking.