Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone baking mats.
In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium-high speed until the mixture is light, fluffy, and pale in color.
Add the egg and vanilla extract to the butter mixture, beating well until fully combined and smooth.
In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt to ensure even distribution of the leavening agent.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a cohesive dough forms and no streaks of flour remain.
Divide the dough into two disks, wrap them tightly in plastic wrap, and refrigerate for at least 45 minutes. This prevents the cookies from spreading and helps them hold their Halloween shapes.
On a lightly floured surface, roll out one disk of dough to a consistent 1/4-inch thickness.
Use Halloween-themed cookie cutters—such as ghosts, pumpkins, and bats—to cut out shapes. Place the cutouts 1 inch apart on the prepared baking sheets.
Bake for 8 to 11 minutes, or until the edges are set and just barely beginning to turn golden. Do not overbake; the cookies should remain soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely before decorating.
For the icing, whisk together the powdered sugar, milk, and vanilla bean paste in a small bowl until smooth and glossy. Adjust consistency with more milk or sugar if needed.
Divide the icing into separate bowls and stir in the food coloring. Use a piping bag or a small spatula to decorate the cooled cookies with spooky designs.