1. Whisk together flour, cocoa powder, and salt in a medium bowl. Beat together butter and sugar with a mixer until pale and fluffy. Beat in eggs, 1 at a time, and vanilla. Reduce speed to low, and gradually add flour mixture. Divide dough in half. Form each half into a disk, and wrap each in plastic wrap. Refrigerate until firm but still pliable, about 1 hour.

2. Preheat oven to 350 degrees. Working with 1 disk of dough at a time, roll out to 1/4-inch thickness on lightly 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 cookies to parchment-lined baking sheets, and freeze until firm, about 15 minutes. Bake until crisp, 12 to 14 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, uncovered, at room temperature, at least 4 hours or (ideally) overnight.