1. Combine nuts, 1/4 cup flour, brown sugar and cinnamon in small bowl. Mash in 2 tablespoons butter with fork or rub in with fingertips until texture of coarse meal. Chill until needed.
2. Preheat oven to 400 degrees F. Grease 12-cup muffin tin; also lightly grease area around tops of muffin wells to prevent sticking as muffins rise.Prepare Batter:
3. Whisk together egg, sour cream, milk, vanilla and melted butter in bowl just until blended. Gently stir in mango.
4. Stir together 2 cups flour, granulated sugar, baking powder, baking soda and salt in large bowl; make well in center. Add mango mixture; fold together just until moistened and evenly blended; the batter will be lumpy.
5. Quickly fill muffin cups to top. Crumble topping evenly over batter.
6. Bake in 400 degree F oven for 25 to 30 minutes or until wooden pick inserted in centers comes out with a few crumbs clinging to it. Cool muffins in pan on wire rack for about 5 minutes. Remove the muffins and cool on rack. Serve warm or at room temperature.