Preheat the oven to 350 degrees and line an 8-inch square pan with parchment paper.
Melt the butter in a medium bowl, then stir in the sugar until smooth.
Add the eggs and vanilla extract, mixing well after each addition.
Stir in the cocoa powder, flour, and salt until no dry streaks remain.
Spread the batter evenly in the prepared pan and bake for 20 to 22 minutes, until a toothpick comes out with a few moist crumbs.
Let the brownies cool completely in the pan before lifting them out and cutting into 4 large squares.
Spread a thin layer of black frosting over each square.
Place two candy eyes near the top of each square and add short pieces of licorice below them to form whiskers.