Whisk together flour, baking powder, and salt. Beat butter and sugar on medium-high speed until pale and fluffy, about 3 minutes. Beat in egg, zest, and flour mixture. Divide dough in half; wrap in plastic. Freeze until firm, 20 minutes.Advertisement
Preheat oven to 325 degrees. Unwrap 1 half on a lightly floured surface. Let stand 10 minutes. Roll out to 1/4 inch thick. Cut out circles with a 2-inch round cookie cutter. Transfer cookies to parchment-lined baking sheets. Roll out scraps; cut out circles. Repeat with remaining dough.
Lightly press 1 end of a clean 1 1/2-inch spool onto surface of each cookie. Refrigerate dough on sheets 10 minutes.
Sprinkle sanding sugar into imprint on cookies; brush off excess. Bake until golden, about 14 minutes. Cool on sheets on wire racks.
Store airtight up to 1 week.