In the bowl of a stand mixer fitted with the paddle attachment, cream the butter until smooth. Add the vanilla extract then half of the confectioners' sugar and beat until combined, scraping down the sides as needed.
Add the heavy cream then the remaining confectioners' sugar and continue beating until the frosting is thick and creamy. (If the frosting is too thick, add more heavy cream as needed. If it's too thin, add more sifted confectioners' sugar.)
To make the chocolate spiders, line a baking sheet with wax paper. Melt the candy melts according to package instructions then transfer them to a piping bag or sealable plastic bag.
Snip off the tip from the bag then pipe the spider bodies, heads and legs. Immediately top them with the chocolate sprinkles and candy eyes then set them aside until they've fully hardened.
When ready to assemble, frost the cooled cupcakes with the buttercream then use an offset spatula to carefully release the spiders from the wax paper and place one atop each cupcake.