Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a large mixing bowl, whisk together melted butter and granulated sugar until smooth.
Add eggs one at a time, followed by the vanilla extract. Mix until fully incorporated.
In a separate bowl, whisk together flour, cocoa powder, salt, and baking powder.
Slowly add the dry ingredients to the wet mixture, stirring until just combined. Do not overmix.
Pour the batter into the prepared pan and smooth it out with a spatula.
Bake for 25–30 minutes, or until a toothpick inserted comes out with a few moist crumbs.
Let brownies cool completely in the pan before decorating.
Once cool, slice into 12 squares.
Melt white candy melts or white chocolate chips. Using a piping bag, pipe a white circle in the center of each brownie to form the eyeball.
Place a mini marshmallow or candy eye in the center of each white circle.
Add a pupil using black decorating gel, if desired.
Use red decorating gel to draw small “veins” coming from the white icing to complete the eyeball effect.
Let decorations set before serving.