Butter and flour an 8 inch square dish and set aside.
Whisk together the spices, flour, baking powder and salt. Set aside.
Stir together the molasses, soda and hot water and set aside to cool to room temperature.
Cream together the butter and brown sugar until light and fluffy. Beat the egg into the creamed mixture.
Add the cooled molasses mixture, then the dry ingredients, mixing only until blended. Pour into the prepared pan and bake in the oven until tests done, 30 to 40 minutes.
In the meantime do the banana sauce. In a medium saute pan, combine the butter, cream, brown sugar, cinnamon and rum. Bring to a boil, then reduce the heat and cook for several minutes longer, stirring constantly until the sauce thickens slightly, about 2 minutes longer. Gently stir in the bananas and heat until heated through.
To serve, cut the gingerbread into rectangles and place on dessert plates. Top each portion with some of the banana sauce and then a dollop of freshly whipped cream.