In a large bowl of a stand mixer fitted with a paddle attachment, mix the butter and sugar on high speed for about 3 minutes until well combined.

Add the egg, egg yolk, and vanilla and mix well, scraping down the sides of the bowl.

Add in the flour, salt, and baking powder on low speed, mixing just until incorporated and the dough comes together. Shape the dough into a flat disk and wrap tightly in plastic wrap. Chill for 1-2 hours or up to 3-4 days in the refrigerator.

On a lightly floured surface, roll out the cookie dough to 1/4-inch thick. Cut out using Halloween-shaped cookie cutters and bake on a baking sheet lined with a parchment paper for 12 to 15 minutes, until the edges just barely start to turn golden. Cool completely before decorating.

In a large bowl of a stand mixer, whisk together the meringue powder and powdered sugar, then slowly mix in the water and vanilla. Increase speed to medium and beat until stiff peaks form, around 5 minutes.

Divide the icing into individual bowls for each color and add gel food coloring, a few drops at a time, mixing well until you achieve your desired shades. From there, you can reserve half of each color at a thicker consistency for piping borders, or thin all the icing to flood consistency.

To thin each color to flood consistency, add 1 teaspoon of water at a time and stir well, continuing to add water as needed until the icing is about the same consistency as honey or shampoo and melts into itself within about 5-10 seconds.

Transfer the icing to piping bags fitted with small piping tips or just snip off the end. Decorate the cookies by first outlining the border, then filling in the middle with flood icing. Use a toothpick or scribe tool to fill in any gaps by spreading the icing around, then tap the cookie on the counter a few times to help the icing settle into a smooth, even layer.

Dry cookies at room temperature for 6-8 hours until the royal icing is completely firm before adding additional layers to the design or stacking or freezing the cookies.