Whisk together flour, baking powder, and salt. Stir in seeds.
Beat butter and sugar with a mixer on medium speed until pale and fluffy, about 6 minutes. Beat in egg and vanilla. Reduce speed to low; gradually beat in flour mixture. Divide dough in half. Shape each half into a disk; wrap in plastic. Refrigerate until firm, at least 1 hour or overnight.
Preheat oven to 325 degrees. Roll out dough 1 disk at a time to just under a 1/4-inch thickness on a lightly floured surface. Using templates, cut out letters with a paring knife. Reroll scraps; cut more letters (you'll need 14 D's and 7 A's, total). Freeze on 2 parchment-lined baking sheets until firm, 15 minutes.
Bake until edges are golden, 15 to 18 minutes, rotating sheets halfway through. Let cookies cool on sheets set on wire racks.