Lightly grease a 9x13-inch baking pan or line it with parchment paper.
Melt the butter in a large pot over low heat.
Add the mini marshmallows and stir frequently until completely melted and smooth.
Remove the pot from the heat. Stir in the vanilla extract and salt.
Add the Count Chocula cereal and gently fold until all of the cereal is evenly coated in the marshmallow mixture.
Transfer the mixture to the prepared pan. Lightly grease a spatula or your hands and gently press the cereal mixture into an even layer. Don't pack it down too firmly.
Allow the bars to cool and set for about 30 minutes.
Lift the cereal mixture from the pan and cut into 20–24 rectangular bars.
Melt the green candy wafers according to the package directions.
Dip the top portion of each cereal bar into the melted green candy coating. Allow the excess to drip off, then place on parchment or wax paper.
While the green coating is still wet, add two candy eyes to each bar.
Once the green coating begins to set, use black decorating gel to draw Frankenstein hair, mouths, stitches, and other details.
Allow the decorations to set completely before serving.