Sift together flour, baking soda, and baking powder into a large bowl. Set aside.
Put butter and brown sugar in the mixing bowl, mix with a hand whisk until fluffy. Mix in spices and salt, then egg and molasses. Reduce speed to low. Add flour mixture; mix with a spatula until just combined.
Divide dough in half, wrap each half in plastic wrap, and refrigerate for at least 2 hours.
Preheat oven to 350 degrees F. Roll out dough on a lightly floured work surface to a 0.25-inch thick. Cut into Gingerbread man with a Gingerbread man cookie cutter.
Bake cookies until crisp but not dark, 12 to 14 minutes. Let cool on sheets on wire racks.
When cool, decorate the cookies with Royal icing.
Notes
For best results, use softened butter and room temperature ingredients. Add flour sparingly if the dough is sticky, roll evenly to 0.25-inch thickness, and divide dough before wrapping and chilling.