1. Bake the brownies.
Prepare brownie batter according to package directions. Bake in a 9×13-inch pan until edges are set but center remains slightly fudgy. Cool completely before decorating.

2. Divide the frosting.
Scoop frosting into small bowls. Tint each with a different Halloween color using gel food coloring — green for slime, purple for witches, orange for pumpkins.

3. Frost the brownies.
Spread or swirl the colored frostings across the brownie surface in thick ribbons. You can marble the colors with a knife for a psychedelic monster effect.

4. Decorate.
Scatter Halloween sprinkles generously, then press candy eyes randomly across the top. Let each “monster” find its own expression!

5. Set and slice.
Chill 15 minutes to firm up frosting, then cut into squares or bars. Serve and watch the monsters disappear.