In a large mixing bowl, whisk together flour, baking powder, and salt; set aside. In another bowl, cream together butter and sugar until light and fluffy, about 3-4 minutes. Beat in egg and vanilla, then add dry ingredients until just combined.

Divide the cookie dough into two disks and wrap each tightly in plastic wrap. Refrigerate for at least 1 hour.

Preheat your oven to 375°F (190°C) and line two baking sheets with parchment paper.

Roll out the chilled dough to about ¼-inch thickness, cut shapes, and place them on prepared baking sheets.

Bake in preheated oven for 8-10 minutes until edges are lightly golden. Cool on sheets for 5 minutes, then transfer to wire racks.

In a clean bowl, whisk together warm water and meringue powder until dissolved. Gradually add sifted confectioners’ sugar and cream of tartar, beating until thick and glossy, 5-7 minutes.

Adjust royal icing consistency with more water as needed, using piping bags to decorate cooled cookies. Allow to dry completely for 6-8 hours or overnight.