In a small bowl, combine melted butter, molasses, sugar and egg. Beat until smooth.
In another bowl combine the flour, baking powder and spices and salt. Add the wet mixture to the dry and combine until a thick batter has been made. Do not overmix.
Spoon the batter into the prepared pan and spread and press down dough evenly.
Bake at 350F for about 15-20 minutes. Check after 15. You want the dough to be set (check with a toothpick) but do not overbake as it will be too dry.
Remove from oven and allow to cool. When cooled combine the frosting ingredients and spread over the gingerbread. Sprinkle with colored sugar now, if desired. Refrigerate for an hour, then remove and cut into desired sized bars.