1. 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 as mall plain round tip.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.