A simple, soft, chocolate sponge makes a perfect cupcake base for you to decorate as you wish. You could pipe on chocolate buttercream, using a wide star nozzle, or make a chocolate ganache, gently heating equal quantities of chocolate and double cream together until combined, then setting aside until firm enough to pipe or spread. Before piping, twist the filled icing bag (disposable ones are much easier to deal with) until there's no air in it.
Get the recipe for chocolate cupcakes here