Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan.
In a large bowl, cream together the butter and sugar until smooth. Beat in the molasses and egg. Combine the flour, baking powder, ginger and cinnamon; stir into the molasses mixture.
Spread evenly into the prepared pan.
Bake for 25 to 30 minutes in the preheated oven. Cake is done when it springs back to the touch.
Allow to cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.