In a small bowl, beat butter and sugar until crumbly, about 2 minutes. Beat in egg. Beat in molasses. Combine the flour, baking soda, ginger, cinnamon and salt; add to butter mixture alternately with water, mixing well after each addition.
Transfer to a 9-in. square baking pan coated with cooking spray. Bake at 350° for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack. Cut into squares; garnish with whipped topping.