1. Combine molasses, sugar, and spices in a medium saucepan, and bring to a boil, stirring to dissolve; boil 2 minutes. Transfer mixture to a stand mixer fitted with the paddle. Add baking soda and salt, and beat on low speed until frothy, about 2 minutes. Beat in butter, 1 tablespoon at a time, until thoroughly combined. Beat in a third of the flour, then the egg and remaining two-thirds flour. Increase speed to medium, and beat until mixture is smooth, pulls away from sides of bowl, and is no longer tacky (add more flour, 1 tablespoon at a time, if needed), about 3 minutes. Transfer mixture to a lightly floured surface, and knead, adding flour as needed, until a cohesive dough forms (it should no longer stick to your hands), about 5 minutes. Divide dough in half.

2. Preheat oven to 325 degrees. Roll out half the dough to a little less than 1/4-inch thickness on generously floured parchment. Brush off excess flour, transfer dough on parchment to a baking sheet, and freeze until firm, about 15 minutes. Cut into shapes with cookie cutters, rerolling scraps as needed. Transfer to parchment-lined baking sheets, and freeze until firm, about 15 minutes. Bake until golden on edges, 18 to 20 minutes, rotating once and firmly rapping baking sheet halfway through baking to flatten any bubbles in cookies. Let cookies cool completely on baking sheets set on wire racks.

3. Place icing in a pastry bag fitted with a small plain round tip (Ateco #0, 1, or 2). Pipe outlines and details on cookies. Immediately sprinkle with sanding sugar, and tap off excess. Let set completely at room temperature, at least 4 hours or (ideally) overnight.