Preheat oven to 350°F with racks in upper-third and lower-third positions. Line 2 baking sheets with parchment paper; set aside. Pulse flour, butter, confectioners' sugar, and salt in a food processor until a dough ball begins to form, about 25 pulses.
Turn dough out onto a lightly floured surface; knead until dough is uniform and comes together, about 3 times. Roll out dough to about a 1/4-in. thickness. Using a 2-in. round cookie cutter, cut out cookies, rerolling scraps once (you should have about 40 cookies total). Arrange rounds at least 1/2 in. apart on prepared baking sheets.
Using a knife, gently score each dough round with a light line across middle of cookie. Working with 1 dough round at a time, hold a bench scraper or a spatula over dough in alignment with the scored line, barely touching the dough with scraper. Sprinkle half of the dough round (using the bench scraper as a barrier) with sanding sugar. Then gently press decorations into dough.
Bake cookies until tops are dry but cookies have not started to brown, rotating baking sheets between top and bottom racks halfway through, about 10 minutes. Let cool completely on baking sheets, about 15 minutes.