Beat the softened butter and granulated sugar until creamy and smooth. Add the egg and vanilla extract, then mix until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture and mix until a soft dough forms.
Divide the dough into portions, flatten into disks, wrap, and refrigerate for about 30 minutes until firm.
Roll the chilled dough on a lightly floured surface to about ¼-inch thickness. Keep the dough evenly thick for consistent baking.
Cut the dough into Halloween shapes such as pumpkins, ghosts, bats, and witch hats. Place the cookies on parchment-lined baking sheets.
Bake at 350°F (175°C) for 8–10 minutes, or until the cookies are set and the edges are lightly golden. Avoid overbaking.
Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack and cool completely.
Mix the powdered sugar, milk, vanilla, and salt to make the icing. Divide it into bowls and add your favorite Halloween food colors.
Decorate the cooled cookies with colorful icing, Halloween sprinkles, and candy eyes. Let the decorations set before serving.