In a large bowl, cream butter and sugar until light and fluffy. Beat in molasses and egg until well blended. In another bowl, whisk together the flour, baking soda, ginger, cinnamon and salt; add to creamed mixture alternately with water.
Transfer to a greased 9-in. square baking pan. Bake at 325° for 35-40 minutes or until a toothpick inserted in the center comes out clean. Place on a wire rack.
For caramel sauce, in a small saucepan, combine brown sugar and cornstarch. Stir in water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in butter and vanilla until smooth. Serve with warm cake. Top with whipped cream if desired.