Combine all ingrediants except butter in a small bowl. Mash in butter with a fork till mixture is crumbly. Chill till needed.
Grease a 12cup muffin tin. In a large bowl, whisk together egg, sour cream, milk, vanilla and melted butter. Fold in mango. In another large bowl stir together flour, sugar, baking powder, baking soda and salt. Make a well in dry ingrediants and add mango mixture. Fold together to just moisten and evenly blend. the batter will be lumpy.
Fill muffin tins to tops. Spinkle topping over and bake 25-30 mins. Cool 10 mins and remove from pan.