Preheat oven to 350 degrees F. Line a 9 x 13 pan with foil, leaving an overhang on the ends for easy removal.
In the bowl of a stand mixer, combine the butter and sugar until smooth.
Add all four eggs and mix until fluffy and well-combined.
Add the salt, cocoa, and vanilla, mix until combined.
Add the flour and mix on low speed until mixture is combined. It will be thick.
Push batter into prepared pan and bake for 25 to 30 minutes, until a toothpick inserted in the center comes out clean.
Remove from oven and set aside to cool before frosting. Once cooled completely, cut into 16 brownies.
Divide frosting among 3 small microwavable bowls. Heat frosting as needed: microwave 1 bowl uncovered on High about 10 seconds until frosting can be stirred smooth. If frosting becomes too firm while decorating, microwave on High an additional 5 seconds; stir.
Decorate as directed below, using 5 or 6 marshmallows for each bowl of frosting.
Frankenstein Brownies - Add 3 drops green food color to frosting in 1 bowl; mix well. Top each of 6 brownies with 1 large marshmallow. Spoon 1 tablespoon green frosting over each marshmallow to coat. Decorate with green baking bits for eyes and ears. Use black gel for mouths and centers of eyes. Cut fruit snack to use for hair.
Ghosty Brownies - Top each of 5 brownies with 1 large marshmallow. Spoon 1 tablespoon white frosting over each marshmallow to coat. Use black gel for eyes and mouths.
Spider Brownies - Add 2 drops pink food color and 1 drop blue food color to frosting in 1 bowl; mix well. Top each of 5 brownies with 1 large marshmallow. Tuck gummy worms (3 per side) candies under each marshmallow for legs. Spoon 1 tablespoon purple frosting over each marshmallow to coat. Use orange baking bits for eyes. Use black gel for mouths, centers of eyes and eyebrows.