Preheat the oven to 350 degrees and line a baking sheet with parchment paper.
Whisk the flour, baking powder, and salt in a bowl.
In a second bowl, beat the butter and granulated sugar until light. Add the egg and vanilla extract and mix until combined.
Stir the dry ingredients into the wet mixture until a dough forms. Roll the dough to 1/4-inch thickness on a lightly floured surface.
Cut out shapes with a ghost cookie cutter and place them on the prepared baking sheet. Bake for 8 to 10 minutes until the edges are lightly golden. Cool completely on a wire rack.
In a clean bowl, whisk the powdered sugar, meringue powder, and water until the icing reaches a thick but pipeable consistency. Divide a small portion into another bowl and tint it black with gel food coloring.
Pipe a smooth layer of white icing over each cooled cookie. Let it set for 10 minutes, then use the black icing to add simple eyes and a mouth on each ghost face.