Preheat your oven to 350°F (175°C). Melt the butter and set aside to cool slightly while you prepare the batter.
In a large mixing bowl, combine the melted butter and granulated sugar. Whisk until the mixture looks smooth and starts to look glossy; this helps the sugar begin dissolving for a shiny batter.
Add the eggs one at a time, whisking well after each addition. The mixture should become thick and pale, with a gentle aroma of vanilla filling the air.
Sift in the cocoa powder, flour, baking powder, and salt. Fold these dry ingredients into the wet mixture gently with a spatula, just until combined and no streaks remain.
Pour the batter into a lined or greased 9x9 inch baking pan. Spread it evenly with a spatula, smoothing out the surface for uniform baking.
Bake for about 25 minutes, or until a toothpick inserted into the center comes out with moist crumbs but no wet batter. The edges should be set and slightly pull away from the sides.
Allow the brownies to cool completely in the pan on a cooling rack. This step is key to achieving clean shapes for the ghosts later.
Meanwhile, prepare the frosting by beating powdered sugar, softened butter, vanilla extract, and milk in a bowl until smooth and spreadable. Adjust the milk for desired consistency—should be thick but easy to pipe.
Once the brownies are cooled, spread or dollop the frosting generously on top. Use a spoon or piping bag to create small rounded blobs, shaping them into ghost-like forms with a swirled top and a slightly tapered body.
Use small amounts of black icing to add two eyes and a mouth to each ghost, giving them a spooky but fun expression. Place the decorations carefully to keep the ghosts whimsical.
Serve the ghost brownies once the decorations are set. These treats are best enjoyed at room temperature, with the creamy frosting contrasting against the dense chocolate base.
