First, preheat your oven to 375°F (190°C). 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 in half, wrap each half in plastic wrap, and chill for at least 30 minutes.

On a lightly floured surface, roll out one portion of chilled dough to about 1/4-inch thickness. Use Halloween-themed cookie cutters to cut out shapes and carefully place them on a baking sheet lined with parchment paper.

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

In a large bowl, combine the powdered sugar and meringue powder. Gradually add warm water, one tablespoon at a time, mixing on low speed until the icing reaches a thick, paste-like consistency.

Increase the mixer speed to high and beat for 5-7 minutes, until the icing is glossy and holds stiff peaks. If the icing is too thick, add a few more drops of water; if it's too thin, add a little more powdered sugar.

Divide the royal icing into separate bowls and tint each with different food colorings (orange, black, green, purple) for various Halloween designs.

Once the cookies are completely cool, transfer the tinted royal icing into piping bags fitted with small round tips. Outline the cookies with a slightly thicker consistency of icing.

Immediately fill in the outlined areas with a thinner consistency of icing (flooding consistency) and use a toothpick to spread it evenly to the edges. Allow the flood icing to dry completely, which may take several hours or overnight.

Once the base layer is dry, add decorative details like eyes, stitches, spiderwebs, or other Halloween designs using contrasting icing colors and Halloween sprinkles. Allow all decorations to dry completely before storing.