Cream the butter in a bowl, and then add the chocolate with the mixer on low speed. Mix in the milk, vanilla extract, confectioners’ sugar and cocoa powder, being careful not to overmix.
To decorate the cupcakes
Ice the cupcakes with plenty of buttercream, creating a very round shape.
Quickly dunk the cupcakes into a bowl containing the chocolate sprinkles, coating the entire surface of the buttercream. Shape the top of the cupcake with your hand if necessary, to round out the sprinkle-coated icing.
Decorate the spider with the red M&M’s as eyes, and the licorice strings as legs.