sweetened whipped cream (optional) or Cool Whip (optional)
Preheat oven to 350 degrees. Grease a 13x9x2-inch baking pan; set aside. In a large bowl combine flour, the 2 cups sugar, baking powder, soda, nutmeg and salt. Using a pastry blender or two knives, cut in butter until mixture resembles coarse crumb.
In a small bowl combine buttermilk, eggs and vanilla. Add to flour mixture all at once; stir just until moistened. Fold in mangoes. Spread in prepared pan. Combine the 1/3 cup sugar and the cinnamon. Sprinkle evenly over batter.
Bake for 40 to 45 minutes or until a wooden pick inserted near center comes out clean. (Mango pieces will sink to the bottom of coffee cake.) Cool slightly. Serve with sweetened whipped cream or Cool Whip, if desired.