Preheat your oven to 350°F (175°C) and line your baking pan with parchment paper for easy removal.
Mix the brownie box ingredients according to the package instructions in a large bowl until smooth and glossy, releasing a rich cocoa aroma.
Pour the batter into the prepared pan, spreading it evenly with a spatula to create a smooth surface.
Bake for about 25 minutes or until a toothpick inserted into the center comes out with just a few moist crumbs. Let the brownies cool completely in the pan before handling.
While the brownies cool, prepare the frosting by beating softened butter with powdered sugar, milk, and vanilla extract until creamy and fluffy, about 3-5 minutes.
Spread the vanilla frosting evenly over the cooled brownies with a spatula, creating a smooth, white surface for the ghost faces.
Fill a piping bag or plastic sandwich bag with the frosting and pipe small ghost shapes or faces onto the decorated brownies.
Add candy eyes or chocolate chips onto each ghost face to give them an expressive, spooky look.
Let the decorated brownies sit for about 10 minutes to set the frosting, then cut into squares and serve your adorable ghost treats.