Preheat the oven to 375°F and grease a 9×9 inch square baking pan with cooking spray and set aside.
In a small bowl, combine the finely chopped lemon peel and sugar. Set aside.
In a medium mixing bowl, whisk together the dry ingredients: flour, salt, and baking powder. Set aside.
In the bowl of a stand mixer, cream the butter. Then add the sugar mixture and blend well together. Add the egg, milk, and vanilla extract. Mix until combined.
Fold in by hand, the coconut and mango chunks.
Pour batter into the prepared cake pan and bake for 25 to 30 minutes.
Remove from oven and allow the cake to cool in the pan for 15 minutes on a wire rack. Frost when completely cooled.
Prepare the frosting: In a large mixing bowl, combine the entire tub of frosting and the lemon extract. Using a hand-held mixer, mix the frosting for a minute or two; then fold in (by hand) the small diced mango pieces.
Frost the cooled cake and then garnish with more mango pieces, if desired. Store the cake covered and in the refrigerator.