Preheat oven to 375°. Line a 12 cup muffin tin with papers.
Stir streusel ingredients together in a small bowl. Set aside.
Stir flour, sugar, baking powder, cinnamon and salt together in a medium mixing bowl. In a separate, smaller bowl, stir together milk, oil and vanilla. Add to dry mixture and stir just until blended. Fold in mangoes and nuts.
Divide batter into paper-lined muffin cups and sprinkle tops with streusel. Bake until muffins spring back when lightly touched and streusel is nicely browned, about 20 minutes.
Remove from oven and transfer to a cooling rack to cool before removing muffins from tin.