Preheat your oven to 350°F (175°C) and line a baking pan with parchment paper for easy removal.
In a large mixing bowl, whisk together the melted butter and sugar until well combined and slightly shiny.
Add the eggs one at a time, whisking thoroughly after each addition to incorporate air and create a smooth, glossy batter.
Stir in the vanilla extract, then sift in the cocoa powder, flour, baking powder, and salt. Fold gently until the mixture is uniform and free of streaks.
Pour the batter into the prepared baking pan, spreading evenly with a spatula to smooth the surface.
Bake in the preheated oven for 20-25 minutes, until a toothpick inserted in the center comes out with moist crumbs and the top is set.
Remove the pan from the oven, let it cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.
Once cooled, cut the brownies into squares or rectangles, creating a flat surface for decorating.
Fill a piping bag or a plastic zip-top bag fitted with a small round tip with white frosting or melted marshmallows.
Squeeze out the frosting onto each brownie as a dollop for the ghost's body, creating a rounded head and body with slight uneven edges for a spooky effect.
Use a small spoon or the back of a knife to slightly smear or melt the edges of the frosting, giving the ghosts a wispy, haunted appearance.
Optionally, sprinkle a few chocolate chips onto the frosting as eyes or allow some to melt slightly for expressive ghost faces.
Place the decorated brownies on a serving platter and enjoy this fun, spooky treat with a glass of milk or hot cocoa.
