Whisk together flour, baking powder, and salt in a bowl. In a separate bowl, cream together butter and sugar until light and fluffy, then beat in the egg and vanilla extract.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Divide the dough into two disks, wrap them in plastic, and chill for at least 30 minutes.

Preheat oven to 375°F (190°C). Roll out chilled dough on a lightly floured surface to 1/4 inch thickness and cut out shapes with Halloween cookie cutters. Bake for 8-10 minutes until edges are lightly golden, then cool completely on a wire rack.

In a large bowl, combine powdered sugar and meringue powder. Gradually add warm water, beating on low speed until combined, then increase speed to high and beat for 5-7 minutes until stiff peaks form.

Divide the royal icing into separate bowls and add food coloring to achieve desired Halloween colors, such as orange, black, green, and purple.

Once cookies are completely cool, transfer the colored royal icing to piping bags with small tips. Outline and flood the cookies with icing, allowing each layer to dry before adding details.

Decorate with Halloween sprinkles, candy eyes, or other edible decorations before the icing sets completely to create spooky designs.