Make and Chill the Dough:
In a large bowl, beat the softened butter and sugar with a mixer until light and fluffy, about 2-3 minutes. Add the eggs one at a time, beating well after each addition. In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients on low speed until just combined. Do not overmix. Divide the dough in half, flatten each into a disk, wrap tightly in plastic, and chill in the refrigerator for at least one hour.

Roll, Cut, and Bake:
Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. On a lightly floured surface, roll one disk of chilled dough to a ¼-inch thickness. Use a ghost-shaped cookie cutter to cut out the cookies and carefully place them about an inch apart on the prepared baking sheets. Bake for 8-10 minutes, or until the edges are firm and just beginning to turn a light golden color. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Prepare the Icing and Decorate:
In a clean, grease-free bowl, beat the egg whites and cream of tartar with a mixer until foamy. Gradually add the powdered sugar while mixing on low speed. Once all the sugar is incorporated, increase the speed to high and beat for 5 minutes until the icing is glossy and forms stiff peaks. Transfer the majority of the white icing to a piping bag. Outline and then “flood” (fill) the surface of each cookie, using a toothpick to spread the icing to the edges. Let the base layer harden for at least one hour.

Add the Final Details:
Divide the remaining icing into small bowls and use gel food coloring to create your desired colors for the faces. Transfer the colored icing to piping bags with small tips (or use a toothpick) to add eyes and mouths to your ghosts. Let the cookies dry completely at room temperature for at least 4-6 hours, or until the icing is fully hardened.