Combine butter and hot water in a large bowl, stirring with a whisk until the butter melts. Add molasses and egg, and stir with a whisk until blended. Lightly spoon flour into dry measuring cups, and level with a knife. Combine flour, baking soda, ginger, cinnamon, salt, and cloves. Add flour mixture to the molasses mixture, stirring just until moist.
Spoon batter into a 9 x 9-inch cake pan coated with cooking spray. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool gingerbread in the pan on a wire rack for 15 minutes.
Combine sugar and lemon juice, stirring until well blended. Pierce top of gingerbread liberally with a wooden skewer. Pour glaze over gingerbread. Cool completely.
Top each serving with the whipped topping. Sprinkle gingerbread with cinnamon and garnish with lemon slices, if desired.