Preheat oven to 350°.
Coat a 9-inch round cake pan with cooking spray; line bottom of pan with wax paper. Coat wax paper with cooking spray.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, salt, baking powder, and baking soda in a bowl, stirring with a whisk. Combine banana and next 5 ingredients (banana through egg) in a bowl; beat with a mixer at medium speed for 1 minute or until well blended. Add flour mixture to banana mixture, and beat until blended. Pour batter into prepared pan.
Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Carefully peel off wax paper.
Combine brown sugar, water, and butter in a small saucepan; bring to a boil. Cook 1 minute, stirring constantly. Remove from heat; stir in nuts and coconut. Spread over cake. Serve cake warm.