PREHEAT OVEN TO 400F.
For topping: Combine all ingredients except butter in a small bowl; mash in butter with a fork or rub in with your fingertips; chill until needed.
Grease a 12-cup muffin tin; be sure to lightly grease area around the muffin wells so they won't stick as they rise. 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 the center of the dry ingredients and add mango mixture. Fold together just to moisten and evenly blend; the batter will be lumpy.
Using a large spoon, quickly fill muffin wells with batter to the tops. Crumble topping over and bake 25 30 minutes or until a toothpick inserted into center emerges with just a few crumbs clinging to it. Cool in pan 5 10 minutes; loosen muffins and tilt each on its side to cool further. Serve warm or at room temperature.
YIELD: 12 muffins