Lightly spoon the flour into a dry measuring cup and level with a knife. Combine the flour and brown sugar in a small bowl. Using a pastry blender or 2 knives cut the butter into the flour mixture until it resembles a coarse meal. Stir in coconut and pecans. Set aside.
For the cake
Coat a 12-cup Bundt cake pan with cooking spray. Sprinkle 1 tablespoon granulated sugar on the inside of bundt pan. Set aside.
Combine cocoa powder and baking chocolate in a small bowl. Add the boiling water and stir until chocolate melts. Set aside.
Using an electric mixer at medium speed, add the granulated sugar and butter to a large bowl and mix for 5 minutes, until well blended.
Add the vanilla and egg whites, 1 at a time, beating well after each addition.
Lightly spoon the flour into dry measuring cups and level with a knife. Combine the flour, baking powder, baking soda, and salt, stirring well with a whisk.
With the electric mixer at low speed, add flour mixture to the sugar mixture alternately with the buttermilk, beginning and ending with flour mixture. Stir the melted chocolate mixture.
Spoon half of batter into the prepared pan and top with streusel.Spoon the remaining batter over streusel and smooth the top with the back of a spoon, being sure to cover the streusel mixture.
Bake at 325° for 1 hour or until a wooden pick inserted in center comes out clean.Cool in pan on a wire rack 10 minutes. Remove the cake from the pan and cool completely.
To prepare glaze (optional)
Combine powdered sugar and melted butter in a small bowl. Add the milk and stir with a whisk until smooth.Drizzle glaze over top of cake.