Lightly spray a 9×13-inch pan with nonstick spray and set aside.
In a large pot over low heat, melt the butter and dissolve the salt.
Add the mini marshmallows and stir until completely melted and smooth. Remove from the heat.
Stir in the vanilla extract. Then fold in the Rice Krispies until evenly coated.
Transfer the mixture to the prepared pan. Use lightly buttered or damp hands to gently press the treats into an even layer. Don’t compact them, or the bars will turn dense.
Let cool for 1 hour, then cut into circles.
Spread a smooth layer of white frosting over the top of each Rice Krispie treat.
Add black frosting to a piping bag fitted with a small round tip. Pipe two large oval eyes and two small nostrils.
Pipe a curved smile near the bottom of the face, then add short vertical lines across it to create Jack Skellington's stitched mouth.
Refrigerate for 10–15 minutes, or until the frosting is set, before serving or packaging.