Heat oven to 400°F. Line a 12-muffin pan (standard-size) with paper baking cups; set aside.
To prepare the muffins, combine the melted butter, coconut milk, and eggs in bowl; beat with a whisk until smooth. Add the flour, sugar, baking powder, and salt; stirring just until moistened. Gently stir in the coconut flakes and chopped mango. Spoon about 2 to 2-½ Tablespoons of the batter into the muffin cups.
In a small bowl, combine all streusel ingredients and mix well. Sprinkle about 1/2 Tablespoon over each muffin.
Bake for 18-20 minutes or until golden brown on top and a toothpick inserted in the center comes out clean. Let cool in pan for about 3-5 minutes. Meanwhile, prepare the coconut glaze if desired. For this, combine coconut milk and powdered sugar in a small bowl, and whisk very well until obtaining a homogeneous, smooth, thick mixture (without lumps). Drizzle on top of the baked mango coconut streusel muffins while still warm. Enjoy!
Store muffins in airtight container in the fridge for up to 3-5 days. Freeze for up to 2 months. Thaw in the fridge.