Combine flour, sugar, salt, ginger and baking soda; set aside. In a bowl, beat egg, molasses, water and butter until smooth. Gradually add the dry ingredients; beat for 1 minute. Pour into a greased 8-in. square baking pan. Combine the sugar and cinnamon; sprinkle evenly over gingerbread.
Bake at 350° for 25 minutes or until a toothpick inserted in the center comes out clean. Cool completely before cutting. Top each square with whipped topping.