Heat the oven to 300°F with a rack in the middle position. Line a rimmed baking sheet with kitchen parchment. In a medium bowl, whisk together the flour, 1 tablespoon of the coriander and the salt.

In a stand mixer with the paddle attachment, beat the butter and powdered sugar on medium until light and fluffy, about 1 minute. Add the orange zest and beat until incorporated, about 10 seconds.

With the mixer on low, gradually add the flour mixture and beat until incorporated and the dough begins to form a ball, about 1 minute.

Lightly dust the counter with flour. Scrape the dough onto the counter and knead by hand for 1 minute. Set the dough on a sheet of kitchen parchment and form it into a 10-by-6-inch rectangle about a scant ½ inch thick, using a rolling pin to even out the thickness and a bench scraper to square the edges.

Slide the parchment with the dough onto the parchment-lined baking sheet (double lining the pan) and refrigerate for 15 minutes.

In a small bowl, stir together the white sugar and remaining 2 teaspoons coriander. Sprinkle the mixture evenly over the dough. With the tines of a fork, poke the dough in rows spaced about ½ inch apart.

Using a paring knife, score the rectangle in half lengthwise, then crosswise into tenths, dividing the rectangle into twenty 3-by-1-inch cookies.

Bake until the shortbread is lightly browned, 45 to 50 minutes. Let cool on the baking sheet on a wire rack for 5 minutes, then cut completely through the score marks to separate into 20 cookies. Let cool completely.