Preheat oven to 325 degrees. Grease a 9 inch round or square pan and set aside. (I used a 9 inch springform pan and it worked great)
In a large bowl or kitchenmaid mixer, cream butter and sugar until light and fluffy. Beat in molasses and egg until blended together. Combine the flour, baking
soda, ginger, cinnamon and salt; add to creamed mixture alternately
Pour the cake into the 9 inch pan. Bake for 35-40 minutes or until toothpick in center comes out clean. Place on a wire rack.
To make the caramel sauce:
In a medium size saucepan, combine brown sugar and cornstarch. Add the water and bring to a boil until the sauce starts to thicken about 2 minutes. Remove from heat and stir in the cubed butter and vanilla until melted and smooth. Serve over warm cake with whipped cream.