Preheat oven to 200°C.
Sift flour, baking powder and salt together.
Add coconut, sugar and rind and mix well.
Beat egg, milk, oil and lemon juice together and gradually add to dry ingredients until flour is moist.
Mix batter lightly.
Spoon into a greased muffin pan and bake for 20 - 25 minutes.
Remove from oven and allow muffins to cool slightly before removing from pan.
Brush each muffin with melted butter and sprinkle with castor sugar.