Preheat your oven to 350° and set a rack to the middle position. Grease and flour a 9-inch square baking pan and set aside.
In the large bowl of a standing or handheld mixer, cream the butter and brown sugar until fluffy, about 2 minutes. Add the egg and continue beating. Add the molasses and beat well. In a medium-size bowl, whisk together the flour with the baking soda, spices, and salt. Add the dry mixture to the wet ingredients; then add the hot water. Stir gently until the batter is just smooth.
Pour the batter into the prepared pan, and bake until the edges begin to pull away from the sides of the pan and a toothpick inserted into the center comes out clean, about 40 minutes. Transfer to a wire rack to cool; then cut into squares and serve warm, topped with powdered sugar or fresh whipped cream.