Halve the mango each side of the flat central stone. Cut away the stone, then peel and dice the flesh into 5 mm (¼ inch) pieces.
Sift the flour and baking powder into a bowl, then stir in the caster sugar and oats. Beat together the egg, milk, vanilla and melted butter in a jug. Add to the dry ingredients with half the mango and stir together using a large metal spoon until just combined.
Spoon the mixture into a greased and lined 1 kg (2 lb) or 1.3 litre (2¼ pint) loaf tin (see
Leave to cool in the tin for 5 minutes, then loosen at the ends and transfer to a wire rack to cool. Peel off the lining paper and serve warm or cold, sprinkled with vanilla sugar.