Whisk baking powder, cardamom, salt, and 2¾ cups (320 g) rice flour in a medium bowl. Process butter and powdered sugar in a food processor until very smooth and creamy. Add egg, egg yolk, and rose water and pulse until smooth. Add about half of dry ingredients; pulse just to combine. Add remaining dry ingredients and pulse until a soft dough forms. It should be moist but firm. Pat into a disk about 1″ thick. Wrap in plastic and chill at least 4 hours and up to 1 day.

Let dough sit at room temperature 20 minutes. Place racks in upper and lower thirds of oven; preheat to 300°.

Roll out dough on a lightly floured piece of parchment paper to about ¼” thick, dusting with more rice flour as needed to keep from sticking. If using stamp, dip in flour and press into dough, then punch out cookies with lightly floured cutter. Arrange on 2 parchment-lined baking sheets, spacing 1″ apart. Sprinkle with toppings as desired.

Bake cookies, rotating baking sheets top to bottom and front to back halfway through, until set around the edges, 8–10 minutes. Let cool on baking sheets.

DO AHEAD: Cookies can be baked 1 day ahead. Store airtight at room temperature.