With the rack in the middle position, preheat the oven to 180°C (350°F). Line two baking sheets with parchment paper.
In a bowl, combine the flour, baking powder, spices and salt. Set aside.
In another bowl, cream the butter, brown sugar and molasses with an electric mixer. Add the egg and beat until smooth and creamy. Using a wooden spoon, mix in the dry ingredients and bananas.
With a spoon, drop 45 ml (3 tablespoons) of dough per cookie on the baking sheets, spacing them 5 cm (2 inches) apart. Sprinkle with sugar if desired. Bake one sheet at a time until the edges of the cookies start to turn brown, about 15 minutes. Let cool before removing from the cookie sheet.