In a large mixing bowl, whisk together 3 cups of all-purpose flour, 1 teaspoon of baking powder, and ½ teaspoon of salt.

In another bowl, cream 1 cup of softened unsalted butter with 1 cup of granulated sugar until light and fluffy, about 3-5 minutes.

Beat in 2 eggs and 2 teaspoons of vanilla extract until fully combined.

Gradually mix in the flour mixture until a dough forms. Shape the dough into disks, wrap them in plastic wrap, and chill for at least 2 hours.

Once the dough is chilled, preheat your oven to 350°F (175°C). On a well-floured surface, roll the dough out to about ¼ inch thick.

Use a round cookie cutter to cut out shapes for your Jack Skellington Cookies. Place the shaped cookies on a parchment-lined baking sheet and freeze them for 15 minutes.

Bake the cookies in the preheated oven for 10 to 12 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.

Prepare the royal icing by combining 4 cups of powdered sugar, 3 egg whites, and 1 teaspoon of cream of tartar in a mixing bowl.

Beat the mixture until slightly thickened, then increase speed until you achieve a smooth, glossy texture that holds peaks.

Transfer the royal icing into piping bags and carefully pipe icing onto each cookie, creating the Jack Skellington design.

Allow the icing to dry completely, which can take about 1-2 hours.