1. Preheat oven to 375. Make the cookies: Whisk together flour, baking powder, and salt in a medium bowl; set aside.

2. Put butter, granulated and confectioners' sugars, vanilla seeds, and zest into the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy. Mix in egg. Reduce speed to low. Add flour mixture in 3 batches; mix until combined.

3. Roll out dough between 2 pieces of floured parchment paper to 1/8 inch thick. Refrigerate until firm, about 45 minutes.

4. Remove parchment. Using a 3-inch umbrella cutter, cut out shapes. Reroll scraps, and cut out. Space cookies 2 inches apart on parchment-lined baking sheets. Bake until pale golden, 5 to 6 minutes. Transfer cookies to wire racks to cool.

5. Make the royal icing: Put confectioners' sugar, meringue powder, and 1/4 cup water into the bowl of an electric mixer fitted with the whisk attachment; beat on low speed 10 minutes. Transfer to a pastry bag fitted with a small plain round tip.

6. Decorate each cookie with icing, and then immediately sprinkle with sanding sugar.