In a medium bowl, whisk together the flour, baking powder, and salt. In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.

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

Preheat oven to 375°F (190°C) and line baking sheets with parchment paper. On a lightly floured surface, roll out one disk of dough to about 1/4 inch thickness and cut out shapes using Halloween-themed cookie cutters.

Carefully transfer the cut-out cookies to the prepared baking sheets. Bake for 8-10 minutes, or until the edges are lightly golden. Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

While the cookies cool, prepare the royal icing by beating powdered sugar and meringue powder with water until stiff peaks form. Divide the icing into separate bowls and tint with gel food coloring to create various Halloween colors.

Once the cookies are completely cool, decorate them with the royal icing using piping bags and desired tips. Add Halloween sprinkles if desired, and let the icing set completely before storing.