Coat an 8-inch square pan with cooking spray. Arrange mango slices in 3 rows in pan. Sprinkle brown sugar over mango. Drizzle with 1/4 cup melted butter; set aside.
In a medium bowl, stir together granulated sugar, egg, and molasses with a wooden spoon. Combine flour and next 3 ingredients; stir into molasses mixture. Add water and remaining 1/4 cup melted butter; stir well.
Spoon batter over mango slices.
Bake at 350° for 35 to 40 minutes or until cake begins to pull away from sides of pan and center is set. Cool in pan on a wire rack 15 minutes.
Beat whipping cream and vanilla at high speed with an electric mixer until foamy; gradually add powdered sugar, beating until soft peaks form.
Serve sweetened whipped cream with warm cake. Store leftovers in refrigerator.
Tip: If fresh mango isn't in season, check the produce section for sliced mango in a jar.