Preheat Oven: Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.
Make Brownie Batter: In a bowl, whisk melted butter and sugar. Add eggs and vanilla. In another bowl, mix cocoa powder, flour, salt, and baking powder. Gradually add to wet ingredients and stir until smooth.
Bake: Pour batter into the prepared pan and bake for 20-25 minutes, or until a toothpick comes out with a few moist crumbs. Let cool completely.
Make Frosting: Beat butter until creamy. Add powdered sugar, cream, and vanilla. Mix until smooth. Add purple food coloring until the desired shade is reached.
Frost and Decorate: Spread frosting evenly over cooled brownies. Top with candy eyes, sprinkles, and mini chocolate chips to create monster faces.
Slice and Serve: Cut into squares and enjoy your spooky, delicious treats!