Step 1

Sift together flour, baking soda, and baking powder into a large bowl. Set aside.

Step 2

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.

Step 3

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.

Step 4

Bake cookies until crisp but not dark, 12 to 14 minutes. Let cool on sheets on wire racks.

Step 5

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.