Step 1: In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer until the mixture is light and fluffy.
Step 2: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a soft dough forms. Be careful not to overmix.
Step 5: Divide the dough into two equal disks, flatten slightly, wrap tightly in plastic wrap, and refrigerate for a minimum of 30 minutes, or up to 2 days.
Step 6: Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper.
Step 7: On a lightly floured surface, roll out one disk of chilled dough to approximately ¼-inch thickness. Use your favorite Halloween-themed cookie cutters (ghosts, bats, pumpkins, etc.) to cut out shapes.
Step 8: Carefully transfer the cut-out cookies to the prepared baking sheets, leaving about 1 inch of space between each cookie.
Step 9: Bake for 8-10 minutes, or until the edges appear lightly golden. Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely. Repeat with the remaining dough.
Step 10: Prepare the royal icing: In a large bowl, whisk together the sifted powdered sugar and meringue powder. Gradually add ⅓ cup warm water, mixing on low speed with an electric mixer until a paste forms. Increase speed to medium-high and beat for 5-7 minutes until the icing forms stiff, glossy peaks. Add more water, a teaspoon at a time, if a thinner consistency is needed for flooding.
Step 11: Divide the royal icing into several small bowls and tint each with different gel food colorings (black, orange, green, purple, etc.). Transfer the colored icing to piping bags fitted with small round tips, or use Ziploc bags with a tiny corner snipped off.
Step 12: Decorate the completely cooled cookies. Begin by outlining each cookie shape, then flood the interior with a slightly thinner consistency icing. Once the base layer is dry, add details such as faces, stitches, spiderwebs, and attach candy eyes or sprinkles. Allow the decorated cookies to dry completely for several hours (or overnight) before stacking or storing.