Sift together flour, baking soda, and baking powder into a large bowl. Set aside.
Put butter and brown sugar in the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until fluffy. Mix in spices and salt, then eggs and molasses. Reduce speed to low. Add flour mixture; mix until just combined. Divide dough into thirds; wrap each in plastic. Refrigerate until cold, about 1 hour.
Preheat oven to 350 degrees. Roll out dough on a lightly floured work surface to a 1/4-inch thick. Cut into snowflakes with a 7- inch snowflake-shape cookie cutter. Space 2 inches apart on baking sheets lined with parchment paper, and refrigerate until firm, about 15 minutes.
Bake cookies until crisp but not dark, 12 to 14 minutes. Let cool on sheets on wire racks.
Put icing in a pastry bag fitted with a small plain round tip (such as Ateco #7). Pipe designs on snowflakes; immediately sprinkle with sanding sugar. Let stand 5 minutes; tap off excess sugar. Let icing set completely at room temperature, about 1 hour.
Sanding sugar atop piped royal icing gives it an icy sheen.
