Step 1

Preheat oven to 350°. Toast pecans on a rimmed baking sheet, tossing once or twice, until lightly browned and beginning to smell nutty, 5–8 minutes (do not toast them all the way; they will get baked again). Let cool. Turn off oven.

Step 2

Pulse pecans, buckwheat flour, salt, baking powder, and 1 cup all-purpose flour in a food processor until pecans are finely ground.

Step 3

Using an electric mixer on medium–high speed, beat butter, powdered sugar, and vanilla until light and fluffy, about 4 minutes. Reduce speed to low and mix in dry ingredients just to blend. Divide dough in half; wrap each in plastic wrap, flattening into ½”-thick disks. Chill until very firm, at least 2 hours.

Step 4

Working with 1 piece at a time, roll disks of dough between 2 lightly floured sheets of parchment paper to about ⅛” thick. Transfer to a baking sheet and remove top sheet of parchment. Chill 30 minutes.

Step 5

While dough is chilling, mix granulated sugar, sesame seeds, and poppy seeds in a small bowl. Reheat oven to 350°.

Step 6

Using a knife or pastry cutter, cut out 2” squares or diamonds from dough and space out on baking sheet, about ½” apart. Sprinkle with sesame mixture and bake cookies until golden brown around the edges, 10–12 minutes. Transfer cookies to wire racks and let cool before serving.

Step 7

DO AHEAD: Dough can be made 5 days ahead; keep chilled. Cookies can be baked 2 days ahead; store airtight at room temperature.