Preheat the oven to 180°C/fan 160°C/gas mark 4. Line a 12-hole muffin tin with 12 paper muffin cases. Sift the flour, baking powder and most of the cinnamon into a large mixing bowl.
In a separate bowl, whisk together the egg, yogurt, oil and 4 tbsp maple syrup until smooth. Stir in the chopped apple. Make a well in the centre of the dry ingredients, then pour in the liquid mixture. Using a fork, stir until just combined.
Spoon the mixture equally into the paper cases, then sprinkle the remaining cinnamon over the top. Bake for 20 minutes, or until pale golden and a skewer inserted into a muffin comes out clean. Transfer to a wire rack and leave to cool.
Sift the icing sugar into a bowl and stir in the remaining maple syrup and 1-2 tbsp water to make a smooth, runny mixture. Drizzle the icing over the cooled muffins using a teaspoon. Allow the icing to set before serving.