Sift the flour and baking powder into a bowl. Stir in the orange rind, oatmeal and sugar.
Beat the yogurt with the oil, milk and egg and add to the bowl of dry ingredients with the chocolate. Using a large metal spoon, carefully fold the ingredients together until only just combined, adding a little extra milk if the mixture seems dry.
Line a 10-section deep bun tin or muffin tin with paper muffin cases. Divide the mixture among the cases and sprinkle some extra oatmeal over the top. Bake the muffins in a preheated oven, 200°C (400°F), Gas Mark 6, for 15–20 minutes until risen and just firm. Serve warm or cold.