Preheat your oven to 350°F (175°C). Line a baking pan with parchment paper for easy removal. In a large bowl, whisk together the cocoa powder, flour, and sugar to combine thoroughly.
Pour in the melted butter, then add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until the batter is smooth and glossy, with a thick but pourable consistency.
Pour the brownie batter into the prepared pan, spreading it evenly with a spatula. Smooth the surface so it bakes evenly. Bake for about 20–25 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Remove the brownies from the oven and let them cool in the pan for about 15 minutes. Transfer them to a wire rack and allow to cool completely before decorating.
While the brownies cool, prepare the marshmallow ghosts. Using a piping bag or plastic sandwich bag with the tip cut off, pipe small mounds of marshmallow fluff or softened marshmallows into ghost shapes on a baking sheet lined with parchment.
Once the brownies are fully cooled, cut them into small, even squares using a sharp knife. Arrange the brownie pieces on a serving platter.
Place a marshmallow ghost on top of each brownie square. Use a toothpick or a small spatula to gently shape the marshmallow into a ghost form if necessary.
Mix powdered sugar with a little milk to create a simple glaze or frosting. Spread a small amount over the marshmallow ghosts to help them stick and add a finishing white, fluffy appearance.
Optionally, use mini chocolate chips or edible black markers to add eyes and smiles to the marshmallow ghosts, giving them a spooky, smiling face.
Refrigerate the decorated brownies for about 30 minutes to let the marshmallow ghosts set. Serve at room temperature for best texture and appearance. Enjoy your creepy, cute Halloween treat!
