Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. In a large mixing bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 3 minutes. Then, add in the egg and vanilla extract, mixing until fully combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually incorporate the dry mixture into the wet mixture, stirring until a soft dough forms.

Shape spoonfuls of the dough into ghost figures on the prepared baking sheets, placing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are lightly golden.

Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.

Roll out the white fondant and cut out ghost shapes. Place the fondant on top of the cooled cookies and draw eyes and mouths with the edible marker.

Dust the finished cookies with confectioners' sugar for an eerie effect and let them set before serving.