Pre-heat the oven to 350 F Degrees. Using 1 tablespoon of butter, generously butter a 9-inch tube pan.
To make the streusel topping, mix the flour, 3/4 cup brown sugar, 1-teaspoon cinnamon, and salt in a medium size bowl. Add in the butter. Using a fork or with a pastry cutter, mash them all together until small clumps form. Stir in the 1/2-cup walnuts. Place it in the fridge until you are ready to use it.
Continue with making the streusel for the center: Mix the remaining 1/4 cup brown sugar, 1/4 teaspoon cinnamon, and 1 cup walnuts. Set aside.
To make the cake batter: Sift the flour, baking soda, baking powder, and salt in a large bowl.
Using a standing mixer with the paddle attachment (or hand mixer), beat the butter and sugar in medium speed, until fluffy, 2 minutes. Add eggs one at a time, making sure each egg is fully incorporated into the batter. Add in the vanilla extract.
Add in the flour in 3 batches, alternating with sour cream, beginning and ending with flour. Mix until just incorporated.
Using a spatula, gently fold in the mashed mango and the shredded coconut into the batter.
Pour half of the batter into the bottom of the tube pan. Use a rubber spatula to smooth out the surface. Evenly sprinkle the streusel you prepped earlier for the center and then cover with remaining batter spreading out evenly.
Sprinkle the streusel topping mixture evenly over the batter.
Bake until the top is golden brown or a skewer inserted into middle of cake comes out or with a few crumbs attached, about 60-65 minutes. Cool the cake in pan on wire rack for half an hour, then invert onto wire rack to cool completely before glazing, about 1 hour.
To make the glaze, mix together the powdered sugar, milk, and vanilla extract in small bowl.
Drizzle it over the cake allowing it to drip down the sides. Allow it to sit for 5 minutes and serve.