Add the butter to a large heavy-bottomed pot set over medium-low heat. Once the butter has melted, stir in the marshmallows and cook, stirring, until the mixture is smooth.
Add food coloring until the mixture reaches your desired color then stir in the cereal until well-coated. Immediately pour the mixture into a 13x9-inch baking pan. (The pan does not have to be greased.) Flatten the cereal into an even layer then let the treats cool completely, about 15 minutes.
Once the treats have cooled, invert them onto a cutting board and cut them into 12 or 18 rectangles.
Break each pretzel stick in half, and then insert each half into the sides of the treats to form the "bolts" in Frankenstein's neck.
Melt the chocolate per the package instructions until smooth. Dip the tops of the treats into the melted chocolate, and then immediately top with sprinkles.
Use the remaining chocolate to adhere the candy eyes and pipe mouths. Let the treats set until the chocolate has fully hardened, about 10 minutes, then serve.